The Basic Forces That Make Weather • Clouds • Fog • Highs & Lows • Air Masses & Fronts • Storms & Severe Weather • Forecasts & How to Get Information • Weather Observing & Weather Proverbs
Weather is an important element of your boating; it can add to or detract from your enjoyment, and it can have a vital bearing on your safety.
A boater needs three different types of weather information: current or “nowcast” conditions showing what the weather is actually like; “forecast” conditions of how the weather is expected to change over a given period lasting from an hour or two to several days; and climatic or “normal” conditions used in planning to determine the type of weather usually to be expected during your time on the water. You need not be qualified as a weather forecaster, but you must be able to obtain up-to-date information and apply that information to your plans. You must be alert to and correctly interpret local weather signs—wind shifts, changes in cloud patterns, or ocean swells.
What Makes the Weather
Weather doesn’t just happen; there are basic causes and effects. An appreciation of these influences will lead to a better understanding of weather systems and their movement. This is the basis for weather forecasting.
The Earth’s Atmosphere
The earth is enveloped in its ATMOSPHERE—a constantly changing, fluid-like mixture of gases, mostly nitrogen and oxygen—that extends upward with decreasing density for many miles. Half of all this air is in the lower 3.5 miles (5.6 km). From a weather standpoint, we are usually concerned only with that portion which lies below about 10 miles (16 km).
The atmosphere consists of several layers. The lowest layer (TROPOSPHERE) is the most important for weather. The TROPOPAUSE at the top of the troposphere generally acts as a cap separating tropospheric from higher air.
The height of the troposphere varies with latitude, averaging 11 miles (18 km) at the equator but only 5 miles (8 km) above the poles. This is the area that contains our weather systems, and most of our clouds.
Heat from the Sun
Although the sun is some 93 million miles (150 million km) away, and only an infinitesimal fraction of its energy falls on the earth; this is enough to make the earth habitable and establish our climate and weather patterns. About 29 percent of the sun’s energy is reflected back into space by the atmosphere. The atmosphere absorbs 19 percent, and 52 percent is soaked up by the earth. Of that 52 percent, energy will be absorbed or reflected depending on the surface at a location: dark or light, rough or smooth, land or water, plant covered or barren. These differences result in the climatic variations seen around the earth. Solar radiation that is absorbed by the earth’s surface is reradiated as heat rays of longer wavelength. Depending on cloud cover, some of this energy can be trapped in the lower atmosphere in much the same way air in a greenhouse is warmed to grow plants—hence the much talked about “greenhouse” effect; see Figure 24-01.
The atmosphere moderates the sun’s effect, filtering out excess (and harmful) rays by day, and holding in heat at night. Without our atmosphere, we would have to cope with extremes like those on the moon, +212° Fahrenheit (F) or more in sunlight, -233° F or less in darkness (+100 to -147° Celsius [C]). Clouds directly affect the cooling of the air at night as they reflect back a portion of the heat rising from the earth’s surface; on clear nights, more heat is lost.
Figure 24-01 The sun’s rays encounter layers of gases in the atmosphere that either stop the rays, reflect them, or let them through. Only about half of the sun’s radiated energy reaches the earth’s surface, where it is reflected or absorbed and partially reradiated.
CLOUDS
The most visible manifestation of weather is clouds; winds and temperature can be felt, but not seen. Many cloud patterns are not only beautiful and interesting to watch, but they can also be meaningful in interpreting weather conditions and trends.
DESCRIBING WATER VAPOR CONTENT
• Condensation A process in which a gas changes to a liquid, as from steam to water.
• Dew point The temperature at which air would become saturated if cooled at constant pressure.
• Evaporation A process in which a liquid turns to a vapor without its temperature reaching the boiling point.
• Relative humidity The actual amount of water vapor in the air divided by the amount of water vapor that would be present if the air were saturated at the same temperature and pressure. It is expressed as a percentage.
Dew point indicates indirectly the amount of water vapor present, while relative humidity expresses the degree of saturation. Therefore, air that has a temperature and dew point of 75°F respectively contains more water vapor than air with a temperature and dew point of 45°F and 45°F respectively. However, the relative humidity is higher in the latter case.
• Saturation The point at which air will not absorb more moisture. The higher the temperature, the more water vapor the air can hold before it becomes saturated.
• Sublimation Conversion directly from a solid to a vapor or vice versa without going through the liquid state.
Moisture in the Air
There is more water in the air than you might think; it evaporates from the oceans, lakes, and rivers and can exist in the air in any of three physical states—vapor, liquid, and solid. A term you will often hear is RELATIVE HUMIDITY—the amount of water vapor present in the air as a percentage of the maximum possible amount for air at that temperature. This maximum amount decreases with a decrease in temperature. The relative humidity of a mass of air increases as its temperature falls, even though the actual amount of moisture is unchanged. When air is cooled after the relative humidity reaches 100 percent, some moisture condenses into a visible form. The temperature at which this occurs is the DEW POINT.
Formation of Clouds
When air moves upward (e.g., moving over mountains, moving over domes of colder air, or moving upward in cumuloform clouds), it expands and cools. As it cools, the relative humidity increases until the air becomes saturated and clouds are formed. There may also be precipitation in the form of RAIN (liquid) or SNOW, SLEET, or HAIL (solid). (When this saturation occurs at or very near the surface, this is FOG. The various types of fog and the circumstances of their formation are discussed in detail later in this chapter.)
Cooling can also come with horizontal air movement. For example, air cools when moving from warmer water surfaces to cooler land surfaces. Again, when air cools to the dew point, clouds form and precipitation can fall.
Types of Clouds
There is an international system of cloud classification. Cloud types have names based mainly on appearance. They may sometimes be classified by the process of formation. One having “cirro” in its name is a high cloud. One with “alto” is a middle cloud. One with “nimbo” is a precipitating cloud. A cloud with “strato” in its name is layered. One with “cumulo” has vertical development. Ten basic types are seen in Figures 24-02 (a-u).
High Clouds
HIGH CLOUDS are found above about 18,000 to 20,000 feet (5,500 to 6,000 m) and are composed of ice crystals. Their various types are described below; see Figures 24-02, a–f.
Cirrus (Ci) Detached clouds in the form of white, delicate filaments, patches, or narrow bands. Often called “mares’ tails,” these clouds have a fibrous (hair-like) appearance, or a silky sheen, or both. Experienced boaters will often use wispy cirrus clouds as the first sign that an approaching cold front will arrive in about 24 hours.
Cirrocumulus (Cc) Thin, white patch, sheet, or layer of cloud, without shading, composed of very small elements. These elements are in the form of grains or ripples, merged or separate, and more or less regularly arranged. Most of the elements have an apparent width of less than one degree of arc.
Cirrostratus (Cs) Transparent, whitish cloud veil of fibrous (hair-like) or smooth appearance, totally or partly covering the sky, and generally producing halos around the sun or the moon.
Middle Clouds
MIDDLE CLOUDS are found from about 7,000 feet up to 18,000–20,000 feet (2,000 up to 5,500– 6,000 m). These are water droplet clouds and are described below; see Figures 24-02, g–i.
Altocumulus (Ac) White or gray (or both white and gray) patch, sheet, or layer of cloud, generally with shading, composed of layers, rounded masses, rolls, etc., which are sometimes partly fibrous or diffuse, and which may or may not be merged. Most of the regularly arranged small elements have an apparent width of between one and five degrees of arc.
Altostratus (As) Grayish or bluish cloud sheet or layer of striated, fibrous, or uniform appearance, totally or partly covering the sky, and having parts thin enough to reveal the sun at least vaguely, as through ground glass. Altostratus does not show halo phenomena.
Low Clouds
LOW CLOUDS, described below, are found from near ground up to about 7,000 feet (2,000 m); see Figures 24-02, j–l.
Nimbostratus (Ns) Gray cloud layer, often dark, the appearance of which is rendered diffuse by more or less continuously falling rain or snow, which in most cases reaches the ground. These clouds are thick enough throughout to blot out the sun. Low, ragged clouds frequently occur below the layer, with which they may or may not merge.
Stratocumulus (Sc) Gray or whitish (or both gray and whitish), patch, sheet, or layer cloud. These almost always have dark parts, composed of tessellations (covering without gaps), rounded masses, or rolls, which are nonfibrous (except for “virga”—precipitation trails) and which may or may not be merged. Most of the regularly arranged elements have an apparent width of more than five degrees of arc.
Stratus (St) Generally gray cloud layer with a fairly uniform base, which may yield drizzle, ice prisms, or snow grains. When the sun is visible through the cloud, its outline is clearly discernible. Stratus does not produce halo phenomena (which is an ice crystal cloud effect) except, possibly, at very low temperatures. Sometimes stratus appears in the form of ragged patches.
Clouds of Vertical Development (Cumuloform)
As opposed to the clouds described above, which normally form in sheets or layers, there are clouds that have significant vertical extent. These develop with a significant rising action; see Figures 24-02, m–t.
Cumulus (Cu) Detached clouds, generally dense and with definite outlines, developing vertically in the form of rising mounds, domes, or towers, of which the bulging upper part often resembles a cauliflower. The upper sunlit parts are mostly brilliant white; their bases are relatively dark and horizontal. Sometimes cumulus is ragged.
Cumulonimbus (Cb) Heavy and dense cloud, with a considerable vertical extent in the form of a mountain or huge towers. This is a thunderstorm; see Figure 24-02s. Depending where on the earth they form, the tops of these clouds may extend to over 50,000 ft (15.2 km). At least part of the upper portion is usually smooth, or fibrous or striated, and nearly always flattened. This part of a cumulonimbus cloud often spreads out in the shape of an anvil or vast plume. Under the base of this cloud, which is often very dark, there are frequently low ragged clouds, either merged with it or not, and precipitation.
Variations
It should be noted that cloud groupings as above are not absolute or exclusive. For example, cumulonimbus often become midlevel altocumulus or altostratus, while the tops of these are high-level cirrus clouds
Figure 24-02a Cirrostratus covers the entire sky, with some thicker patches of altostratus below it. As cirrus clouds thicken, the lower portions become water droplets.
Figure 24-02b Cirrocumulus cloud puffs are smaller in apparent size than the sun or moon, which they would not obscure.
Figure 24-02c Cirrostratus is a high veil of ice crystal cloud through which the sun can be seen, along with a well-defined halo; it may herald an approaching storm system.
Figure 24-02d Ground haze, mixed with industrial smog, dissipates in morning sunlight, which is not blocked by a thick layer of cirrostratus cloud.
Figure 24-02e Cirrocumulus clouds are heavier and thicker than cirrus, but still without shading. Small elements may be more or less arranged in repeating patterns.
Figure 24-02f Cirrus are high, thin clouds, either in white filaments or patches and narrow bands. They may have a fibrous (hair-like) appearance or a silky sheen, or both.
Figure 24-02g Altocumulus—white or gray, or both—often appears in regular patterns across large areas of the sky. The pattern above is called a “mackerel sky.”
Figure 24-02h Altostratus represents warm air riding over cold air ahead of a warm front. The layer will grow thicker and lower with time, bringing continuous rain or snow.
Figure 24-02i More altostratus that covers the entire sky, with widely scattered stratocumulus beneath it. This followed a day of warm, hazy sunshine in late May.
Figure 24-02j Nimbostratus, just prior to the onset of a steady rainfall. If the sun were in the picture, its image would be blurred and there would be no halo.
Figure 24-02k Stratus has a thick, almost uniform look. This indicates stable air, and as there is little turbulence in this, any precipitation will be fine drizzle.
Figure 24-02l Stratocumulus is white with dark patches in thick portions of the clouds. This type may appear as rounded masses or rolls that may or may not be merged.
Figure 24-02m Cumulus of little development indicate continued fair weather; following the passage of a cold front, they may be accompanied by brisk winds.
Figure 24-02n More fair-weather cumulus. If they form in warm air ahead of a cold front, however, they can be accompanied by squalls and thunderstorms.
Figure 24-02o Cumulonimbus base as a line of heavy thunderstorms approaches. Squall winds reached 50 knots in gusts, and the accompanying rain was torrential.
Figure 24-02p Cumulus clouds normally are associated with fair weather, but local hot spots and extreme development can change them into storm cumulonimbus.
Figure 24-02q Cumulus may build in the afternoon, but later in the day will flatten as the surface cools and the supply of rising warm, moist air diminishes.
Figure 24-02r Stratocumulus is a thick, solid layer with a lumpy base. It is often formed by the spreading out of cumulus and may be followed by clearing at night.
Figure 24-02s Cumulus development culminates in an anvil-topped cumulonimbus thunderhead that may tower 20,000 to 50,000 feet above the surface of the earth.
Figure 24-02t Cumulonimbus associated with summer showers results from local developments of cumulus clouds. Rainfall is likely to be brief, but moderately heavy.
Figure 24-02u Altostratus here is a thick, almost uniform cloud layer about 10,000 feet high. If this thickens and lowers, precipitation is on the way.
FOG
Fog is merely a cloud whose base rests upon the earth, either land or water. It consists of water droplets that are suspended in the air. Each droplet is so small that it cannot be distinguished individually, yet present in such tremendous numbers that objects close at hand are obscured; see Figure 24-03.
If we are to have innumerable water droplets suspended in the air, there must be much water vapor already in that air. If droplets are to form from this vapor, the air must somehow be cooled so the vapor will condense. If the droplets are to condense in the air next to the earth, the cooling must take place at the surface of the earth. If the fog is to have any depth, successively higher layers of air must be cooled sufficiently to allow condensation in them also. Fog usually forms from the surface up. Thus, the land or water must be colder than the air next to it; and this must be colder than air above.
Air is said to be SATURATED with water vapor when its water vapor content would remain unchanged if it were placed above a level surface of pure water at its own temperature—it could absorb no more moisture. The amount of water vapor required to saturate a given volume of air depends on the temperature of the air, and increases as the temperature increases. (Remember, the higher the temperature, the more water vapor the air can hold before it becomes saturated.)
If a mass of air is originally in an unsaturated state, it can be saturated by cooling it down to a temperature at which its content of water vapor is the maximum containable amount, that is, to the DEW-POINT TEMPERATURE. Or it can become saturated by more water being caused to evaporate into it until it can hold no more. In this latter process, unsaturated air, as it passes over rivers and lakes, over the oceans or over wet ground, picks up water vapor and has its relative humidity raised. Also, rain falling from higher clouds will increase the amount of water vapor in unsaturated air near the earth.
Figure 24-03 Fog forms when warm, moist air is cooled to the point where the water vapor in it condenses out. Fog forms from the surface up; as successive layers of air are cooled enough to cause condensation, the depth of the fog increases.
Types of Fog
As a boater, you can be affected by four types of fog: (1) RADIATION FOG, formed in near-calm conditions by the cooling of nearby land on a clear night as a result of radiation of heat from the ground to the clear sky; (2) ADVECTION FOG, formed by the flow of warm air over cold sea or lake; (3) STEAM FOG, or SEA SMOKE, formed when cold air blows over much warmer water; and (4) PRECIPITATION FOG, formed when rain coming out of warm air aloft falls through a shallow layer of cold air at the earth’s surface.
Note that both steam fog and rain fog are basically the result of evaporation from relatively warm water, a process that increases the dew point. These fogs can be described as WARM-SURFACE FOGS. On the other hand, radiation fog and advection fog can be considered COLD-SURFACE FOGS.
There are also two other types of fog, but they are ones less likely to be a problem in boating: these are commonly referred to as UPSLOPE FOG and ICE FOG.
Radiation Fog
There are four requirements for the formation of radiation fog (sometimes called GROUND FOG). First, the air must be stable, and the air next to the earth must be colder than the air a short distance aloft. Second, the air must be relatively moist. Third, it must be night and the sky must be clear so that the earth readily can lose heat by radiation to outer space. The fourth requirement is that the wind must be light to calm.
The third requirement of the clear night sky enables the ground to become colder than the overlying air, which subsequently is cooled below its dew point both by contact with the ground (conduction of heat) and by radiative loss of heat to the ground. The characteristics of the fog developed vary with the degree of the fourth requirement of calmness. If there is a dead calm, the lowest strata of air will not mix with the ones above, and fog will likely form only to a height of 2 to 4 feet (0.6 to 1.2 m). If there is a slight motion of the air (a wind of three to five knots) and, hence, some turbulent mixing, the cooling is spread through a layer that may extend to a height of several hundred feet (perhaps 125 m) above the ground. With stronger winds, the cooling effect is distributed through so deep a layer that temperature does not fall to the dew point and fog does not form.
Radiation fog is most prevalent in middle and high latitudes; it can be predicted with considerable accuracy;. It is local in character and occurs most frequently in valleys and lowlands, especially near lakes and rivers where you might be cruising. The cooled air drains into these terrain depressions; the lake or river aids the process of fog formation by contributing water vapor, which raises the dew point. Often there is dense radiation fog in narrow rivers and channels leading to a large body of water that is absent of fog away from shore.
This type of fog may be patchy or uniformly dense. It affects boaters mainly in the late summer or early autumn. Having formed at night, it will usually start to evaporate (“burn off”) over land shortly after sunrise. The lower layers are the first to go. It is slower to clear over water, however, since the temperature of the water warms less from night to day than does the land.
Advection Fog
Whereas radiation fog bothers boaters chiefly in the late summer and early autumn, advection fog can be a problem in any season. Advection means “transport by horizontal motion,” which relates to the production of this type of fog by winds carrying warm, moist air over a colder surface. The dew point of the air must be higher than the temperature of the surface over which the air is moving. The air can be cooled below its dew point by conduction and by radiation of heat to the colder surface. Other requirements for the formation of advection fog are: (a) the air at a height of 100 feet (30 m) or so must be warmer than the air just above the surface; and (b) the temperature of the surface—be it land or water—must become progressively colder in the direction toward which the air is moving. Advection fog may form day or night, in any season of the year, especially over the sea.
Coastal Fog For boaters, the most bothersome variety of advection fog is COASTAL FOG. When steady winds blow landward and carry warm oceanic air across cold coastal water, the resulting fog may blanket a great length of coastline and, especially at night, may extend many miles inland up bays and rivers. It can be seen on land as it blows past streetlights. For example, consider the Pacific Coast, where the water close to the land is often colder than the water well offshore. The prevailing winds in summer are onshore and the air (which frequently has come from mid-Pacific) is usually nearly saturated with water vapor. The same thing happens when southerly winds carry air across the Gulf Stream and thence northward across the colder Atlantic coastal waters.
For inland boaters, advection fog may be a problem as it forms over large lakes whenever relatively warm and moist air is carried over their colder surfaces.
Advection fog is generally dissipated less easily than is radiation fog. Unlike radiation fog, sunshine usually has no effect on advection fog over water. Usually, a wind shift or a marked increase in wind velocity is required.
Precipitation Fog
When rain, after descending through a layer of warm air aloft, falls into a shallow layer of colder air at the earth’s surface, there will be some evaporation from the warm raindrops into the colder air. Under certain conditions, this will raise water vapor content of the cold air above the saturation point and precipitation fog, also called RAIN FOG, will form. Because this type of fog is often found near frontal activity, it is sometimes known as FRONTAL FOG.
Steam Fog
Figure 24-04 Cold air passing over warm water picks up enough heat and moisture to form steam fog. It is most prevalent, morning and evening, during autumn months. It is found most often on inland rivers as well as small lakes and ponds.
On large rivers, such as the Mississippi or the Ohio, steam fog can be a particular hazard to late evening or early morning boating in the autumn. When cold air passes over much warmer water, the lowest layer of air is rapidly supplied with heat and water vapor. Mixing of this lowest layer with unmodified cold air above, can, under certain conditions, produce a SUPERSATURATED (foggy) mixture. Because the water is much warmer than the air, vertical air currents are created, and we observe the phenomenon of steaming, in much the same way as steam forms over a hot bath; see Figure 24-04.
Sea Smoke In winter when cold air below about 10°C) blows off the land and across the adjacent coastal waters, steam fog may be widespread and very dense, and is then termed sea smoke; see Figure 24-05. Along North American coastal waters, steam fog occurs most frequently off the coasts of Maine and Nova Scotia, and in the Gulf of St. Lawrence, where it can be a serious navigational hazard. However, its occurrence is not restricted to the higher latitudes. Off the east coast of the U. S. it has been observed as far south as Florida, and it also occurs occasionally over the coastal waters of the Gulf of Mexico.
Figure 24-05 Sea smoke at Great Harbor, Woods Hole, Massachusetts. Water temperature at the time of this photograph was +31.6°C), and the air temperature, 30 feet (9.1m) was +5˚F (-15˚C). The wind was from the northwest at about 20 knots.
Other Types of Fog
When a low, moving blanket of air is gradually elevated toward a cooler altitude by the slope of the land, “upslope fog” forms. Occurring frequently on the sides of mountains, this type of fog is common, for example, at New Hampshire’s Mount Washington and in the Great Plains, where a southeast wind blows moist air from the Gulf of Mexico toward the Rocky Mountains. If winds are too strong, however, stratus or stratocumulus clouds will form instead of fog.
At very low temperatures, usually below -25°F (-32°C), the air may become full of ice crystals. This “ice fog” can seriously restrict visibility. In this case, the water vapor in the air has turned directly into ice through the sublimation process.
Distribution of Fog (Contiguous 48 States)
In the United States, the coastal sections most frequently beset by fog range from the Strait of Juan de Fuca to Point Arguello, California, on the Pacific Coast, and from the Bay of Fundy to Montauk Point, New York, on the Atlantic Coast. On these waters, the average annual number of hours of fog occurrence exceeds 900—more than 10 percent of the year. In the foggiest parts of these areas, off the coast of Northern California and the coast of Maine, fog is present about 20 percent of the year.
Going southward along both the Atlantic and Pacific coasts, the frequency of fog decreases, more rapidly on the Atlantic Coast than on the Pacific. The average annual fog frequency over the waters near Los Angeles and San Diego, for example, is about three times that in the same latitude along the Atlantic Coast.
Seasonal Frequencies
The time of maximum occurrence of fog off the Pacific Coast varies somewhat with the various localities and, of course, with the individual year. In general, however, over the stretch from Cape Flattery, Washington, to Point Arguello, the season of most frequent fog runs from July through October, with more than 50 percent of the annual number of foggy days occurring during these months. However, along the lower coast of California, from Los Angeles southward, the foggiest months are those from September through February, and the least foggy are from May through July.
On the Atlantic side, off the coast of New England, the foggiest months are usually June, July, and August, with a maximum of fog generally occurring during July. In this month, fog is normally encountered about 50 percent of the time. Off the Middle Atlantic Coast, however, fog occurs mostly in the winter and spring months, with a tendency toward minimum frequency in summer and autumn. Along the South Atlantic Coast (from Cape Hatteras to the tip of Florida) and in the Gulf of Mexico, fog rarely creates a problem for boaters. It is virtually nonexistent during the summer, and even in the winter and early spring season (December through March), when it has maximum frequency, the number of days with fog rarely exceeds 20 during this four-month period.
The Great Lakes as a whole tend to have fog in the warmer season. The explanation for this is to be found in the comparison of the lake temperatures with the air temperatures over the surrounding land. From March or April to the beginning of September, the lakes tend to be colder than the air. Hence, whenever the dew-point temperature is sufficiently high, conditions favor the formation of advection fog over the water.
The greatest fogginess occurs when and where the lakes are coldest in relation to the air blowing off the surrounding land. On Lake Superior, north-central Lake Michigan, and northwestern Lake Huron, the time of maximum frequency is late May and June; elsewhere it is late April and May. Since the lake temperatures become colder from south to north and from the shores outward, the occurrence of fog increases northward and toward the lakes’ centers.
WINDS
One of the major weather elements is WIND—the horizontal movement of air. Winds have a major impact on people and their activities, not the least of which is boating.
Circulation Patterns
Wind is explained by a fundamental law of physics. As a gas (in this case, air) is heated, it expands, becoming less dense (lighter), and thus tends to rise. Conversely, cooling air contracts, becoming more dense (heavier) and tends to sink. Air is primarily heated by contact with the earth’s surface; this air rises, and surrounding surface air rushes in to fill the relative void, which is, in turn, replaced by cooler air sinking from aloft. This process of CIRCULATION is essentially continuous, resulting in horizontal and vertical air movements over wide areas.
The nearer the equator, the more nearly vertical the angle of the sun’s rays are all year, resulting in greater heating at the surface. Thus, the equatorial air rises and flows toward the poles at higher altitudes. Rising air is then replaced by a stream of colder air from the poles toward the tropical regions at lower altitudes. This simple pattern is modified by the earth’s rotation (known, for the person who described it, as the Coriolis effect) and by the distribution of landmasses.
In the northern hemisphere, the northward flowing air at high altitudes is bent eastward and, by the time it reaches about 30° latitude, has started to build up an area of higher pressure forcing some of the flow downward and back toward the equator. This portion becomes a steady flow near the surface back toward the equator that is bent westward by the earth’s rotation—the reliable “northeast trade winds” of the northern subtropical zone. The portion that continues northward toward the pole eventually descends as the “prevailing westerlies” of the higher midlatitudes. Between these regions there are the “horse latitudes,” near 30°, where winds are weaker and less constant. Near the equator, where the warmed air rises and turns north or south, the region of weaker winds is called the “doldrums”; see Figure 24-06.
Figure 24-06 These idealized global air circulation patterns are based to a large extent on the heated air that rises along the equator and descends as cold air at the poles and at latitudes approximately 30° from the equator, the “horse latitudes.”
Local Wind Patterns
Temperature differences—heating and cooling air—cause localized breezes as well as global winds. Land areas usually heat up more quickly than water areas during hours of sunlight; air rising over the land is replaced by air coming in from seaward—these are the refreshing “sea breezes”; see Figure 24-07A.
Subsequently, at night, the land loses its heat more quickly, the water’s surface becomes the relatively warmer areas, and “land breezes” flow towards the sea. In both cases, these breezes are felt quite close to the surface; there is a counterflow of air at higher elevations to complete the local circulation pattern; see Figure 24-07B.
Both of these breezes, interacting with general wind patterns, can generate showers and thunderstorms.
Figure 24-07 In the daytime, A, when the land is warmer, air rises over the land and is replaced by cooler air coming in from seaward. This activity creates a “sea breeze.” At night, B, the opposite occurs when the temperature of the land is lower than that of the sea. There is then a “land breeze.”
Winds Afloat
Wind direction and strength are a matter of constant interest, and sometimes concern, to every boater. The measurement of these values, and their use in forecasting immediate and future weather conditions will be covered later in this chapter.
HIGHS & LOWS
Familiarity with atmospheric pressure is essential in understanding weather because the pressure distribution in the atmosphere controls the winds and, to a considerable extent, the occurrence of clouds and precipitation. For a boater watching the weather, it is important to comprehend how the winds and other elements relate to the pressure distribution on a weather map.
The global circulation of the atmosphere resulting from unequal heating results in the building up of areas where the weight of air (atmospheric pressure) is greater than or less than that surrounding it. These are the HIGHS and LOWS that are seen on weather maps, usually designated by large letters “H” and “L.”
General Characteristics
Highs, also called “anticyclones,” generally bring dry weather. Lows, also called “cyclones” (this term is used to generically describe weather events having a strong cyclonic circulation and is also used specifically to describe hurricanes in the Indian Ocean), generally bring unsettled weather and precipitation. Highs and lows are rather large areas of weather measured in many hundreds of miles or kilometers across. In the northern hemisphere, circulation around an area of high pressure is clockwise; around lows, it is counter-clockwise. (Circulation is the opposite in the southern hemisphere.) Winds are generally weaker in highs than in low-pressure systems.
The Formation of Highs
In the earth’s general circulation system, areas of high pressure are formed by the descent of cold, dense air toward the surface in polar regions and in the horse latitudes (near 30° latitude); refer to
Figure 24-06. As air flows outward toward areas of lower pressure, the rotation of the earth changes its direction so that a clockwise circulation is established. Highs form sequentially in the north polar region and move southward; as they reach the latitudes of the prevailing westerlies, they are carried first southeastward, than eastward, and often finally northeastward.
The existence of continents and oceans distorts the theoretical picture of the formation of highs and lows; the actual process is quite complex. High-pressure “breeding zones” form in relatively specific areas rather than in broad zones; these areas change between summer and winter.
The Formation of Lows
The formation of low-pressure cells usually is quite different from that of highs, although large-scale lows, for example, annually develop over deserts and other relatively warm spots. However, the following describes what is usually considered when a low is discussed. The boundary between two masses of air of different temperatures is termed a FRONT. This is a major weather phenomenon that will be considered in detail in the following section. On such a boundary, a horizontal wavelike situation develops; refer to Figure 24-13. This grows and becomes more and more distinct and may even “break” as an ocean wave does on a beach.
AIR MASSES & FRONTS
A general knowledge of air masses and fronts will make weather forecasts more understandable and more useful.
Air Masses
An air mass is any large body of air (covering as much as several hundred thousand square miles (a million or more square kilometers) in which the conditions of temperature and moisture are essentially homogeneous.
Basic Characteristics
Air masses derive their basic characteristics from the surface beneath them. The homogeneous properties are acquired as the mass remains over the SOURCE AREA for an extended period of time until the uniformity is achieved. This source area can be continental or maritime and polar or tropical. Once developed, the mass tends to retain those characteristics even while moving over different surfaces; see Table 24-1. Those characteristics and their comparison with those of the surface over which the air mass is moving—warmer or colder—define the air mass. Figure 24-08 shows the principal air masses that affect North America and their general normal movement.
AIR-MASS CHARACTERISTICS
Observed | Stable | Unstable |
Weather | Air Mass | Air Mass |
Visibility | Poor | Good |
Winds | Steady | Gusty |
Clouds | Stratoform | Cumuloform |
Precipitation | Steady | Showery |
Table 24-1 An air mass is a body of air in which temperature and moisture conditions are essentially the same in all directions horizontally. It may be described as stable or unstable, as shown in the table above.
Maritime and continental air masses differ significantly in their characteristics of temperature and humidity and generate different kinds of weather. Oceans experience less extreme variations of heat and cold than do continents, and maritime air masses change less with seasons. As a result, a maritime air mass moving over land tends to moderate any conditions of excess heat or cold.
Most active weather is generated around lows or at the boundaries between air masses. However, showers or thunderstorms frequently occur within moist air masses, especially warm moist air masses.
Figure 24-08 Shown here are the sources and direction of movement of the air masses that influence weather in North America. The properties of an air mass, and its conflicts with adjacent air masses, are the causes of weather changes. Note that some are seasonal only, such as those above the south-central U.S. (near Texas) called “superior,” meaning it is an exceptionally dry air mass.
Weather Fronts
A general knowledge of air masses is important as it leads directly to our consideration of weather fronts. A front is the boundary between two different air masses; the bodies of air do not tend to mix, but rather each moves with respect to the other. The passage of a front results in a change of weather conditions at that location. The bigger the difference in characteristics between two adjacent air masses, the stronger the front is said to be. And the stronger the front, the greater is the potential for active, sometimes violent, weather.
Cold Fronts
With a COLD FRONT, the oncoming cold air mass, being denser, pushes under the warm air mass and forces it upward; see Figure 24-09. In the northern hemisphere, cold fronts generally lie along a NE-SW line and move eastward or southeastward. The rate of movement is roughly 400-500 statute miles (650-800 km) per day. The stronger the cold front, the faster it tends to move. Since air masses tend to be most diverse in the transition seasons, fronts generally will move the fastest in late winter and spring.
A strong, rapidly moving cold front will bring weather changes that may be quite intense but relatively brief in duration. These active cold fronts are often characterized by a line of strong thunderstorms. In late summer, when air mass differences are smaller, slower moving cold fronts may be accompanied by scattered clouds along the frontal boundary, but no precipitation.
A SQUALL LINE is a line of thunderstorms ahead of an approaching cold front. In fact, there may be more than one squall line in very unstable air masses. Squall lines may be very turbulent with strong winds endangering small craft. A strong increase in wind (called a SQUALL) often accompanies a squall line. Boaters need to understand the difference between GUSTS and SQUALLS. A gust is a rapid fluctuation of wind with a variation of at least 10 knots between peaks and lulls. Gusty conditions may last for several days. A squall, on the other hand, is a sudden increase in which the average wind speed rises at least 15 knots and remains at 20 knots or more for over one minute. Although both are potentially dangerous, the squall, because of the suddenness and intensity of change, is usually worse. A squall line appears as a wall of very dark, threatening clouds. A squall line may produce weather more severe than that of the front itself.
The approach of a cold front is indicated by a shift of the wind toward the south, then to the southwest. Barometric pressure readings fall. As the front approaches, tall cumuloform clouds are usually present with the bottoms (bases) of these dropping closer to the earth’s surface. Rain starts slowly but may increase rapidly.
As the front passes the wind continues to veer (change in a clockwise direction), westward, northwestward, and then sometimes northerly. After passage the sky clears quickly, temperatures drop, pressure builds up quickly, and the wind may continue to veer to the northeast. For a few days at least, the weather will have the characteristics of a cold air mass, although gusty winds may continue, especially if the front was strong.
Figure 24-09 In this cross section of a cold front, a cold air mass is shown advancing from the left, pushing aloft the warm air mass it displaces. Thunderstorms usually occur when the temperature differences are great and rapidly rising warm air ahead of the cold front forms towering cumulus clouds.
Warm Fronts
A WARM FRONT occurs when an advancing warm air mass reaches colder air. The warmer air, being less dense, cannot push under the cold air such as in a cold front. It rides up over it; see Figure 24-10. Warm fronts are generally oriented in directions N-S, NW-SE, or E-W and change their direction more often than cold fronts do. The rate of movement is slower, 150 to 200 miles (240–320 km) per day; thus warm fronts are eventually overtaken by the next following cold front.
Warm-front weather is usually milder than that of a cold front and may extend several hundreds of miles (500–600 km) in advance of the frontal line. Clouds are found at lowering levels as the front approaches, and rainfall is generally more moderate but extended in time. The approach of a warm front is signaled by a falling barometer (but falling more slowly than for an approaching cold front), a buildup of clouds, and the onset of rain or drizzle. The winds are generally from an easterly direction north of the front and gradually veer to the south as the front passes. After the front passes there will be cumulus clouds and temperatures will rise; the barometer will also slowly rise.
Figure 24-10 In this cross section of a warm front, the warm air is shown moving aloft on top of the underlying wedge of cold air, which retreats as the warm air advances. The cloud sequence at the front is first cirrus, then cirrostratus, then altostratus, and finally nimbostratus in the rain area.
Stationary Fronts
Occasionally, when a frontal boundary is between two similar air masses, the front will show little or no forward movement; this is a STATIONARY FRONT. Any weather along the front is likely to be weak.
Occluded Fronts
An occluded front is a more complex situation where we have warm air, cold air, and colder air. It occurs after a cold front, because of its faster movement, overtakes a warm front and lifts the warm air mass off the ground. Either the warm front or the cold front is pushed upward from the earth’s surface; see Figure 24-11 and Figure 24-12. The appearance on a weather map is that of a curled “tail” extending back toward the low from the junction of the cold and warm fronts. In a satellite photo, this looks like a comma. Both warm-front and cold-front precipitation can occur surrounding an occlusion.
Figure 24-11 This cross section of a warm-type occlusion illustrates how the advancing cool air rides up on the cold air ahead—acting somewhat like a warm front—and the warm air then is pushed aloft.
Figure 24-12 In this cross-section of a cold-type occlusion, the advancing cold air moves under the cool air forcing the cool air aloft—acting like a cold front. Once again, the warm air is moved above the surface.
STORMS & SEVERE WEATHER
Although a boater is interested in whatever the weather is and what is forecast, real attention is focused on the possibility of STORMS. While sunny skies and fair winds are eagerly awaited, it is the approach of a storm that causes concern for the safety of the crew, passengers, and for the boat itself.
Extra-Tropical Cyclones
The principal source of rain, winds, and generally foul weather in the U.S. is the EXTRA-TROPICAL CYCLONE. Although some other types of storms may be more destructive, the extra-tropical cyclone is the ultimate cause of most active weather. Such a storm (in the northern hemisphere) is defined as a traveling system of winds rotating counterclockwise around a center of low barometric pressure and containing at least a warm front and a cold front. The warm air mass is typically moist tropical air, and the cold air mass is polar continental.
Development of an Extra-Tropical Cyclone
The development of an extra-tropical cyclone is shown in Figure 24-13. At A, there is a warm air mass, typically moist tropical air, flowing northeastward bordering a cold air mass, typically polar continental, flowing southwestward. They are separated by a heavy line representing the front, a stationary front, between them. At B, a small wave has formed on the front. Cold air from behind pushes under the warm air; the warm air rushes up over the cold air ahead of it. A cold front is born on the left, a warm front on the right. Where they are connected, the barometric pressure is lowered and the air starts circulating counterclockwise. A low develops, and a high will form behind the cold front.
Pushed by the earth’s general circulation patterns, the whole system keeps moving in a general easterly direction. When warm, moist air is lifted, as it is when a cold air mass pushes under it or when it rushes up over cold air ahead, it cools by expansion. After its temperature has fallen to the dew point, excess water vapor condenses to form clouds and then precipitation; the stippled area represents this.
At C in Figure 24-13, the storm is steadily developing, with the low-pressure area intensifying more and more. The clouds and rain are increasing, and the winds are becoming stronger. At D, the storm is approaching maturity. The circulation around the low has intensified.
By the time E is reached, the cold front has begun to catch up with the warm front and an occluded front is formed; the storm is now at its height. When the situation at F is reached, the storm has begun to weaken and the weather will soon clear as a high behind the cold front reaches the area. (Exception: Cold air over warm water may actually enhance the development of showers and thunderstorms.)
Extra-tropical cyclones are constantly forming, moving, and dying around the globe. It takes about one day (24 hours) for any one disturbance to reach maturity. However, although most usually dissipate within three or possibly four days, some can last for a week or more. In winter, these storms occur on the average of twice a week in the U.S.; in summer they occur somewhat less frequently and are less intense. Their movement is generally eastward to north of east at a speed in winter of about 700 miles (1,100 km) per day and in summer perhaps 500 miles (800 km). Such storms usually cover a large area; they can affect a given locality for two days or more.
Figure 24-13 This series of sketches illustrates the development of an extra-tropical cyclone. The thin lines are isobars that pass through points of equal barometric pressure. Arrows indicate wind directions.
Thunderstorms
While extra-tropical cyclones are the principal source of wind, rain, and generally foul weather for large areas, there are also local, small-scale but intense THUNDERSTORMS, which you must be prepared to handle. These are embedded in extra-tropical cyclones, usually linked to the cold and occluded fronts. If the air comprising the warm sector is sufficiently unstable, however, which is often the case in spring or summer, thunderstorms may also be associated with the warm front or be found within the warm sector itself.
A thunderstorm is a storm of short duration, arising only from a cumulonimbus cloud, attended by thunder and lightning, and marked by abrupt fluctuations of temperature, pressure, and wind. A LINE SQUALL, now usually referred to in meteorological terms as a squall line, is a lengthy row of thunderstorms that may stretch for 100 miles (160 km) or more.
Incidentally, a SHOWER (as opposed to gentle, steady rain) is a smaller brother to the thunderstorm, though the rainfall and wind in it may be of considerable intensity. A shower is the product of relatively large cumulus (TOWERING CUMULUS) clouds separated from one another by blue sky. A shower is over quickly and is not accompanied by thunder and lightning. However, it may be strong enough to generate a waterspout.
Requirements for Thunderstorm Formation
There are three requirements for the development of a thunderstorm.
• Lifting Mechanism There must be strong upward air currents, such as those caused by a cold front burrowing under warm air or by the heating of air in contact with the surface of the earth on a summer day, to lift surface air.
• Instability The air parcels forming the storm must be buoyant (warmer) relative to the air outside the storm and be able to keep on ascending higher and higher until they pass the freezing level.
• Moisture The air must have a large concentration of water vapor. The most promising thunderstorm air is of tropical maritime origin; whenever it appears in your cruising area, especially when a cold front is approaching, you need to be watchful.
The life cycle of a thunderstorm usually lasts about an hour. However, subsequent thunderstorms will often develop near where the former one dissipated, feeding off the moisture left by the earlier storm, making it seem as if the storm were longer-lived than it actually was. Also, the cooler, denser air brought to the earth’s surface by the downdrafts induced in the previous thunderstorm can act as the lifting mechanism for generating the next one.
As the thunderstorm cycle begins, a cumulus cloud forms and grows vertically. Boaters should watch such growing clouds and developing storms very closely and be prepared to take evasive or protective actions; see Figure 24-14. Vacuuming air from miles around its base, and feeding on moisture that may have been carried aloft by earlier clouds, the cumulus cloud continues growing. It becomes a towering cumulus and, eventually, it develops into a cumulonimbus (thunderstorm) cloud. Rain drops or ice particles form and, as they become too heavy for the updraft to support, fall, inducing downdrafts. Over time, these downdrafts cut off the air inflow, and the growth process ceases. The cloud essentially precipitates itself away, leaving residual middle- and high-level clouds.
Figure 24-14 This sequence of photographs shows the growth of an anvil top. The left photo was taken at noon, the middle at 1220, and the right photo at 1230.
Characteristics of a Thunderstorm
In all cases, the prime danger signal for a thunderstorm is a cumulus cloud growing larger. Every thunderstorm cloud has four distinctive features, although you may not always be able to see all four as other clouds may intervene in your line of sight. Figure 24-15 is a drawing of a cumulonimbus cloud showing these four features diagrammatically. Starting at the top, notice the layer of cirrus clouds, shaped like an anvil (consequently called an ANVIL TOP), leaning in the direction toward which the upper wind is blowing; this generally tells us the direction in which the storm is moving. The anvil-top development is illustrated in Figure 24-14; these three photographs were taken from the same position over a time interval of only one-half hour!
The next feature is the main body of the cloud—a large cumulus of great height with cauliflower sides. It must be of great height, as it must extend far above the freezing level if the cirrus anvil top is to form. (Cirrus clouds are composed of ice crystals, not water droplets.)
The third feature is the ROLL CLOUD. This is formed by violent air currents along the leading edge of the base of the cumulus cloud.
The fourth and final feature is the dark area within the storm and extending from the base of the cloud to the earth; at the center of this is precipitation. This core is generally beneath the strongest part of the thunderstorm. Hail and heavy precipitation are most likely to be found in this core area.
Ahead of a thunderstorm the wind may be steady or variable, but as the roll cloud draws near, the wind may weaken or become unsteady; see Figure 24-15. As the roll cloud passes overhead, violent shifting winds accompanied by strong downdrafts may be expected. The wind velocity may reach 60 knots or more. Heavy precipitation, and sometimes hail, begins to fall just behind the roll cloud. The weather usually clears quickly after the passage of the storm. The precipitation usually has brought cooler air to the surface so, for at least a time, temperatures and humidity are lower.
Individually, thunderstorms can produce TORNADOES, WATERSPOUTS, or MICROBURSTS with winds measuring over 120 knots. They can spawn hailstones as large as grapefruits and can generate lightning bolts of unbelievable power. The thunderstorm is the single most powerful weather element there is. All severe weather either comes from or, like a hurricane, is made up of thunderstorms. Never take these lightly. When you hear thunderstorms in the forecast or see them in your area, heighten your alert.
Figure 24-15 This typical thunderstorm is based on a cumulonimbus cloud that may tower from 25,000 to 50,000 feet high. The anvil top indicates the direction that the wind is blowing.
Frequency of Thunderstorms
Thunderstorms occur most frequently, and with the greatest intensity, in the spring and summer over all parts of the U.S. They may strike at any hour, but are most common in the late afternoon and early evening over inland and coastal waters. The surrounding land has been a good “stove” for many hours, heating the air to produce strong upward currents. Over the ocean, well away from shore, thunderstorms more commonly occur between midnight and sunrise. Finally, thunderstorms are most frequent and most violent in subtropical latitudes. Boaters in the southeastern part of the U.S. may see four or more thunderstorms per summer week.
If the cumulonimbus cloud is fully developed and towers to normal thunderstorm altitudes, 35,000 feet (10.7 km) or more in spring and summer, the storm will likely be violent. If the anvil top is low, only 20,000 feet (6.1 km) or so, as is usual in winter and autumn, the storm will likely be less severe. If the cumulonimbus cloud is not fully developed, particularly if it lacks the anvil top, and the roll cloud is missing, only a shower is most likely.
Anticipating Thunderstorms
The possibility of a thunderstorm may be first noticed from static crashes on an AM radio receiver. You can plot the approach, once the cumulonimbus cloud is visible, with a series of bearings; you can also estimate its distance from you. The thunder and lightning occur simultaneously at the point of lightning discharge, but we see the lightning discharge much sooner than we hear the thunder. Time this interval in seconds. Multiply the number of seconds by 0.2 (or divide by 5); the result will be approximate distance off in statute miles (multiply by 0.34, or divide by 3, to get distance off in kilometers). Take care, however, that you are properly associating a particular flash with its thunder; this may be difficult if there is nearly continuous lightning.
A good safety guide is the “30/30” rule. The first “30” represents 30 seconds or about 6 miles.
If the time between lightning and thunder is less than 30 seconds, you or your vessel can be struck. The second “30” is to remind the boater to wait 30 minutes after the last lightning flash to move from shelter. More than half of the lightning deaths each year occur after a thunderstorm has passed. Also remember, lightning may impact an area out to 60 feet or more from where it has hit the surface.
Take precautions well before lightning begins. Don’t wait. Lower radio antennas, outriggers, and sails, and drop anchors if necessary. Go into the cabin if possible, or at least keep a low profile below the freeboard. Avoid contact with the hull or with metal fittings, especially those associated with the lightning conduction system. Don’t touch radio equipment or wiring. Depending on your situation, you might want to disconnect power cables from expensive equipment, separating these wires as far apart as possible. And keep your life jacket on in case you are rendered unconscious.
LIGHTNING & THUNDER
A buildup of dissimilar positive and negative electrical charges occurs within a vertically developing cumulonimbus cloud, between the cloud and the earth below (the earth normally has a negative charge), or between neighboring clouds. When the buildup of opposite charges becomes great enough, a LIGHTNING FLASH occurs. These can occur within a cloud, from one cloud to another, or between the cloud and ground. Actually what we think of as a flash is really a series of strikes back and forth over a period of roughly two-tenths of a second. A flash to the surface usually starts with a faint “leader” from the cloud followed instantly by a massive strike upward from the surface. About two-thirds of all lightning flashes are within clouds and never reach the surface. A lightning flash is almost unbelievably powerful—up to 30,000,000 volts at 100,000 amperes; it happens so quickly that it is essentially explosive in nature.
The sudden, vast amount of heat energy released by a lightning flash causes the sound waves termed THUNDER. This release of energy comes from the collapse of the atmosphere around the lightning strike.
Tornadoes
Tornadoes are whirlpools of small horizontal extent that extend downward from a thunderstorm and have a funnel-like appearance; see Figure 24-16. (The only exception: weak tornadoes can be generated when spiraling surface winds are caught in the updraft of a rapidly developing towering cumulus cloud.) The average diameter of the visible funnel cloud is about 250 yards (230 m), but the destructive effects of this system of whirling winds may extend outward from the tornado center as much as one-half mile (0.8 km) on each side. The wind speed near the core can only be estimated, but it is at least as high as 300 knots. Any thunderstorm can produce a tornado. However, the bigger the cumulonimbus cloud, the more likely it is that a tornado will form. Although tornadoes can occur at any time and in any location that a thunderstorm is found, they usually are seen, in boating areas, adjacent to the Atlantic and Gulf of Mexico coasts of North America and in all inland boating areas east of the Rocky Mountains.
Thunderstorms producing tornadoes move with or to the right of the upper-level winds pushing the storm. This wind is usually from the southwest. The air mass in which the storm develops typically consists of two layers: a very moist one (whose source was the Gulf of Mexico) near the ground, and a relatively dry layer above. The temperature decreases with altitude rather rapidly in each layer. When this combination of air layers is lifted along a squall line or cold front, excessive instability develops, and violent updrafts are created.
Figure 24-16 A tornado funnel is a cloud of water droplets mixed with dust and debris that are drawn up because of the greatly reduced atmospheric pressure within the funnel. This pressure drop causes the wind to whirl inward and upward.
Waterspouts
Tornadoes do occur over water. When this occurs, they are called WATERSPOUTS. Although there are other less intense features over the water that are also called waterspouts (similar to dust devils over the desert), boaters should treat any such feature with respect and allow a lot of distance between it and their vessel. They are a threat to small craft. The conditions favoring the formation of waterspouts at sea are similar to those conducive to the formation of tornadoes over land. However, they are much more frequent in the tropics than in middle latitudes.
A waterspout, like a tornado, forms under a towering cumulus or cumulonimbus cloud. A funnel-shaped protuberance (FUNNEL CLOUD) first appears at the base of the parent cloud and grows downward toward the sea. Beneath it the water becomes agitated, and a cloud of spray forms. The funnel cloud descends until it merges with the spray; it then assumes the shape of a tube that stretches from the sea surface to the base of the cloud; see Figure 24-17.
Figure 24-17 A waterspout over St. Louis Bay, off Henderson Point, Mississippi. Note the cloud of spray just above the sea surface. This is the marine equivalent of a tornado, although usually smaller.
The diameter of a waterspout may vary from 20 to 200 feet (6–60 m) or more. Its length from the sea to the base of the cloud is usually between 1,000 and 2,000 feet (300–600 m). It may last from ten minutes to half an hour. Its upper part often travels at a different speed and in a different direction from its base, so that it becomes bent and stretched-out. Finally, the tube breaks at a point about one-third of the way up to the cloud base, and the “spout” at the sea surface quickly subsides.
The considerably reduced air pressure at the center of a waterspout is clearly indicated by visible variations of the water level. A mound of water, a foot (0.3 m) or so high, sometimes appears at the core, because the atmospheric pressure inside the funnel is perhaps 30 to 40 millibars less than that on the surface of the water surrounding the spout. This difference causes the rise of water at the center.
Remember, tornadoes and waterspouts are winds. You cannot see the wind. What you see in each is cloud material that has been sucked down by the whirling action or surface material (water, dirt, or other debris) picked up by the strong winds. Such material may be carried for long distances. For example, considerable quantities of salt spray, picked up by the strong winds at the base of the spout, are sometimes carried far aloft. This has been verified by observations of a fall of salty rain following the passage of a waterspout.
Boaters need to understand that some waterspouts are tornadoes that formed over land and then moved out over water. Special marine warnings are issued to alert boaters of such an occurrence. These are broadcast over VHF NOAA Weather Radio and by the U.S. Coast Guard.
MICROBURST: A DISASTROUS FORCE
Tornadoes are not the only strong winds that can come from a thunderstorm. There is a potential danger lurking in the form of a MICROBURST. This is a concentrated column of sinking air that spreads out in all directions when it reaches the surface. These straight-line winds may reach 30 to 40 knots but in some instances have been estimated at over 120 knots. Microbursts often occur in a series of varying powers and dimensions. Although they last only a few minutes, they are known to have capsized and destroyed boats. The greatest threat is to sailboats underway with normal sails set—the almost instantaneous onset of winds from an unpredicted direction (squall) is a major threat. Unlike a dry squall, which can usually be seen approaching across the water, a microburst’s winds will strike your boat before anyone takes notice. They frequently appear in groups; don’t relax after one passes!
Although microbursts are usually associated with seasonal thunderstorms, they can also occur during rainstorms that are not accompanied by thunder and lightning, such as along squall lines. The strongest winds are in or near the center of the storm where heavy rain, and frequently hail, is falling. Often the gust front precedes the microburst; this is a frontal zone of advancing cold air, characterized by a sudden increase in wind speed and followed by gusty winds. The combination of these two strong wind systems can easily be fatal.
The best protection against microbursts is avoidance. Prepare for every boating trip by obtaining the latest marine weather forecasts. Listen to the NOAA Weather Radio channels or the U.S. Coast Guard. Continuous broadcasts of the latest marine weather information are provided on various frequencies;. And keep aware of what is happening around you. Remember that thunderstorms are dangerous weather systems and can have strong, gusty winds that vary in direction and speed. Be sure your boat has adequate stability to help you cope with strong winds in the event of their unexpected occurrence.
Another phenomenon associated with thunderstorms is the MACROBURST. This is, essentially, a microburst that covers a wider area—more than 2.5 miles in diameter—with generally similar conditions.
Hurricanes: The Tropical Cyclones
HURRICANE is the popular term for a TROPICAL CYCLONE in North America. Unlike an extratropical cyclone, it is not related to warm and cold fronts. A hurricane is defined as a storm of tropical origin with a counterclockwise circulation reaching a strength of 64 knots (74 mph) or more at the center.
In its earlier stages, with winds less than 33 knots (38 mph) but with a closed circulatory pattern, the term is a TROPICAL DEPRESSION. When the winds increase beyond 33 knots, it is a TROPICAL STORM until it reaches hurricane strength. In the Western Pacific Ocean, the term used for tropical cyclones is TYPHOON, and because of the greater expanse of ocean there, these storms often become even larger and more intense than hurricanes. In the Indian Ocean, they are called CYCLONES.
Frequency of Tropical Cyclones
The frequency of hurricanes and typhoons varies around the world. In the Far East, typhoons may occur in any month, although they are most common in late summer and early autumn. In North American waters, the period from early December through May is usually, but not always, hurricane free. August, September, and October are the months of greatest frequency; in these months hurricanes typically, but again not always, form over the tropical Atlantic, mostly between latitudes 8° N. The infrequent hurricanes of June and November almost always originate in the southwestern part of the Caribbean Sea. Interestingly, hurricanes are extremely rare in the South Atlantic.
Hurricanes also occur in the Eastern Pacific off the coast of Mexico, and, infrequently, the direct effect of these storms reaches well into the southwestern U.S. More frequently, the swells generated by these storms can cause much damage to coastal regions of southern California. Some of these storms may affect the Hawaiian Islands.
Development of a Hurricane
The birthplace of a hurricane typically lies within a diffuse and fairly large area of relatively low pressure situated somewhere in the 8°N latitude belt. In the Atlantic, they often develop from weak disturbances moving off the Sahara Desert. The winds around the low-pressure area are not particularly strong, and, although cumulonimbus clouds and showers are more numerous than is usual in these latitudes, there is no clearly organized “weather system.” This poorly defined condition may persist for several days before hurricane development commences. The development of highly sophisticated WEATHER SATELLITES has greatly improved the ability of forecasters to watch vast ocean areas where these storms originate for any signs of conditions favorable for hurricane development.
When development starts, however, it takes place relatively quickly. Within an interval of 12 hours or less the barometric pressure drops 15 millibars or more over a small, almost circular area. Winds increase and form a ring around the area; the width of this ring is at first only 20 to 40 miles (37 to 74 km). Clouds and thunderstorms become well organized and show a spiral structure. At this stage the growing tropical cyclone acquires an EYE. This is the inner area enclosed by the ring of stronger winds, and within the eye we find the lowest barometric reading. As the tropical cyclone intensifies and as the central pressure continues to fall, the features become more defined and the winds speeds increase. By the time the cyclone reaches maturity, the ring of strongest winds surrounding the eye will have reached a diameter of somewhere between 50 (small hurricane) and 300 miles.
Hurricane eyes average about 15 miles (28 km) in diameter but may be as large as 25 miles (46 km). The wind velocities in the eye of a hurricane are seldom greater than 15 knots and often are less. Cloud conditions vary over a wide range. At times there are only scattered clouds, but usually there is more than 50 percent cloud cover. Through the openings, the sky is visible overhead, and, at a distance, the dense towering clouds of the hurricane ring can extend to great heights. This feature is the EYE WALL. Seas within the eye are heavy and confused; the “calm” is only with respect to winds.
Hurricane Tracks
Hurricanes have such an impact that they may seem to move in any direction they choose. However, the usual track of an Atlantic hurricane is a parabola around the semipermanent Azores-Bermuda high-pressure area. Thus, after forming, a hurricane will move westward on the southern side of the Azores-Bermuda High, at the same time tending to work away from the equator. When the hurricane reaches the western side of this high, it begins to follow a more northerly track, and its direction of advance changes progressively toward the right. The position where the westward movement changes to an eastward movement is known as the POINT OF CURVATURE.
Occasionally when a hurricane is in a position near the southeast Atlantic coast, the Azores-Bermuda High happens to have an abnormal northward extension. In this situation the hurricane may fail to execute a complete recurvature in the vicinity of Cape Hatteras. It will skirt the western side of the high and come ashore along the southeast U.S. Coast, as occurred with Hurricane Katrina, a major storm of 2005; see Figure 24-18.
The rate of movement of tropical cyclones while they are still in low latitudes and heading westward is about 15 knots, which is considerably slower than the usual rate of travel of extra-tropical cyclones. After recurving, they begin to move faster and usually attain a forward speed of at least 25 knots. They may reach 50 to 60 knots in the latter stages before dissipating.
Other storms, particularly early in the season, form in the Caribbean Sea and move westward and north into the Gulf of Mexico. And still other hurricanes, like Katrina in 2005, form later in the season.
Hurricanes gradually decrease in intensity after they reach middle and high latitudes and move over colder water. They quickly lose strength after moving on shore. All tropical cyclones lose their identity eventually, becoming large extra-tropical cyclones.
Figure 24-18 After crossing Florida from east to west, Hurricane Katrina strengthened as it moved over the Gulf of Mexico and came ashore on the Mississippi-Louisiana coast with winds of 145 mph, gusting to 160 mph. More than 1,300 persons died; hundreds of ships and thousands of boats were destroyed or damaged.
Effects of El Niño and La Niña
The phenomenon called El Niño is a disruption of the ocean-atmosphere system in the tropical Pacific Ocean. Basically, every few years the water across a huge area in the eastern Pacific warms. This, of course, warms the air above it. The result is that the earth’s entire circulation pattern changes. Of most significance is that this changes the position of the JET STREAM winds (high-speed winds near the tropopause, generally westerly). Since storm systems tend to follow the jet stream, for about a year or so, the usual time period of an El Niño, unusual weather patterns result. Rainfall may be increased in the southern part of the U.S. and Peru, while drought conditions develop in Australia and neighboring.
The existence of El Niño conditions mitigates both the frequency and intensity of Atlantic hurricanes.
La Niña is the opposite condition of below-average sea surface temperatures across the equatorial eastern Pacific. Possible results include an active Atlantic hurricane season, above-average precipitation in northwestern North America, and below-average precipitation in the southern U.S.
WEATHER FORECASTS
Some years ago, to be considered a fully qualified skipper, you had to be able to take a series of complex daily weather maps and forecast your own weather; see Figure 24-19. Now this is no longer necessary, although the ability to read and understand a simplified weather map will always be useful. The ready availability of weather forecasts by radio, TV, satellites, marine facsimile, and the Internet does much of the work for you—but you still have the responsibility to get, and heed, the information.
Although weather forecasting is a science that has improved in recent years, every boater knows that it is far from infallible. Get your forecasts by any and every means possible, but keep an eye on present conditions, especially on changes in them. A forecast of good, safe conditions for your general area does not preclude temporary local differences that could be hazardous. Keep alert for special warnings broadcast by the Coast Guard on VHF Channel 22A (or 22) following a preliminary announcement on Channel 16.
Incidentally, if you see conditions that differ from those forecasted, especially if they are potentially life threatening, try to forward that information to the NWS so they can let others know. There are lots of places where the NWS has no observations; you may be the only “eyes” they have.
Figure 24-19 The symbols shown above are those found on weather maps used professionally by the National Weather Service. The sample station model, Figure 24-20, shows how these are used. The average boater need know only a few of them.
Weather Maps
As with your navigation charts, information on weather maps is presented by use of symbols. Some of the more frequently encountered symbols will be described below; the many others now used only by professionals are omitted; refer to Figure 24-19.
Station Models
A complete STATION MODEL for a reporting location will tell you much more than you need to know; see Figure 24-20. It includes information on wind direction and speed; temperature; visibility; cloud type, amount, and height; current pressure and past change plus current tendency; precipitation in past three hours; dew point temperatures; and current weather. Simplified models omit much of this and often show only current weather, temperature, and wind direction and speed.
Current weather is indicated by the appearance of the circle in the center of the station model—open for clear, partially or totally solid for degrees of cloudiness. Current weather—rain, snow, drizzle, etc.—is shown by a symbol immediately to the left of the station circle. Wind direction is indicated by a WEATHER VANE line and wind speed by the number, size, and shape of the FEATHERS on the end of the weather vane. Temperature will normally be given in degrees Fahrenheit (degrees Celsius in Canada and other areas using the metric system).
Figure 24-20 The sample station model shown here illustrates how the many different symbols depicted in Figure 24-19 are combined to give a complete indication of the existing conditions at a specific location.
Air Masses & Fronts
Air masses may or may not be shown in detail. If their characteristics are shown, large block letters will be used: P for polar and T for tropical. Either of these may be further described as maritime (m) or continental (c). Thus we have cP, mP, and mT—the most common of air masses—as well as cT. Since k indicates colder than the land mass over which it is moving and w indicates warmer than the land mass over which it is moving, more precise labels would include cPk and mPw, for example.
Usually, weather maps show simply highs and lows with a large block letter, H or L; occasionally the word “high” or “low” will be spelled out, or a circle drawn around the letter.
Fronts are shown as a heavy line. Cold fronts have a series of solid triangles on the line that point in the direction of movement; see Figure 24-21. Warm fronts have solid half circles on the line; again the side that they are on indicating the direction of movement of the front. An occluded front has alternating triangles and half circles on the same side of the line. A stationary front has the same alternating symbols but with the triangles on one side of the line and the half circles on the other, indicating no movement.
In some simplified sketches, a cold front is shown by a solid heavy line, whereas a warm front is two parallel fine lines; an occluded front is thus a broken line with alternating solid and open segments. On less detailed maps, a front may be shown as merely a heavy line labeled “Cold” or “Warm,” or “Stationary” front. On weather maps in color, a warm front is normally shown in red, and a cold front in blue.
Figure 24-21 Fronts are shown on weather maps by solid lines, usually heavier than other lines, with triangles or half circles used singly or in combination to indicate the type of front. These symbols face the direction in which the pressure system and its front are moving.
Isobars
A line connecting points of equal barometric pressure is termed an ISOBAR. Such lines are usually drawn on weather maps at intervals of four millibars of pressure—996, 1,000, 1,004, etc.—and are labeled along the line or at each end (96, 00, 04, etc.). On some weather maps, isobars will be labeled for pressure in terms of inches of mercury: 29.97, 30.00, 30.03, etc.
Precipitation
Any precipitation—rain, snow, or hail—is shown by one of a series of symbols; refer to Figure 24-19. On less detailed maps, areas of precipitation are often shown by shading or crosshatching, with a descriptive word nearby or different forms of shading used to distinguish rain, snow, and other forms of precipitation.
Using Weather Maps
Unless your boat has a weather facsimile (fax) machine or access to the Internet, it is unlikely that you would be able to receive a weather map while out cruising. But if you are so equipped, or can get them on shore, you might like to try your hand at making predictions from one or more weather maps.
Predictions from a single map are, of course, less accurate and reliable than those based on a series of weather maps over regular intervals of time. A forecast from a single map can only be made for 6 to 12 hours ahead—there are too many factors that can upset an orderly flow of events. (Make sure that you are using a map of actual conditions, as of a specified recent time; many weather maps now printed in newspapers are for predicted conditions for the day of publication.) Assume that a front is advancing at roughly 20 miles an hour in the normal direction of movement. Sketch in the position of the front 12 hours after the date and time of the map—assuming that your map of actual conditions is recent enough that you still have time in which to make a usable 12-hour forecast. Then with the advanced front, and knowledge of the conditions accompanying this front, you can make your own forecast.
If you have a series of maps of existing conditions at daily or half-daily intervals, you can make a much better forecast. Predict your frontal movement based on trends as well as actual events. If weather trends tend to repeat themselves in your areas, file sets of weather maps for typical frontal passages as additional guidance. Keep a record of your forecasts, and compare them later with the conditions that actually occurred. Do this over a period of time and the accuracy of your predictions is sure to improve. You can also compare your forecasts with those received by radio or TV from professionals—but don’t peek: make your forecasts first, then compare.
Newspaper Weather Maps
The weather maps printed in daily newspapers vary widely in how much information is presented and how details are shown; invariably these maps are less detailed than are official NWS weather maps. Basic information will be shown sufficiently to get a very generalized picture of nationwide weather, but little more than that. As noted above, maps are usually for predicted conditions rather than a report of actual events.
Television Weather Maps
Weather maps shown as part of local or network news programs also vary widely in level of detail. They do have advantages over newspaper maps in that they frequently show current conditions, and color is used for a more vivid presentation. Most TV weather reporters use animation to show predicted movement of fronts and weather patterns. Often satellite photographs, either still photos or time-lapse loops, are shown to explain conditions or trends. Current and recent weather radar scans are also interesting and helpful in visualizing rain patterns out to about 125 miles (230 km).
Many local cable TV systems carry The Weather Channel—a continuous live broadcast of current weather conditions and forecasts. Information is quite comprehensive and several specialized services are included. Weather maps in a number of formats are shown, as well as satellite pictures and radar plots from areas of significant weather activity. Periodically, the national broadcast is interrupted for local reports and predictions.
Companion subscription services, transmitted via satellite to boats and ships, include live weather reports (with radar data updated at five-minute intervals), coastal and offshore forecasts, storm tracking and weather advisories, high-resolution wind and wave graphics, and other valuable information. The data are displayed on the vessel’s multifunction monitor screen. With the appropriate service subscription, the receiver used to capture the satellite broadcast can also provide access to the service’s entertainment programs.
Receiving Weather Forecasts
Professional meteorologists take great care in the preparation of their forecasts. Your responsibility is to read or listen carefully—don’t see or hear something that isn’t there, or something that you were hoping for. Most times on a boat you will get your forecast by listening to a radio transmission. If it is a scheduled broadcast, get a pencil and paper ready and take notes; if you have to use an abbreviated format to keep up with the flow of information, expand your fragments into complete form as soon as possible while the data are still fresh in your mind. Using a small tape recorder can help. If it is a continuous broadcast, take notes anyway, but listen a second time to expand your initial notes. Listen as many times as necessary to get the details fully and accurately. Special warnings on NWS stations are preceded by a ten-second high-pitched tone. If you hear this, grab paper and a pencil and stand by to record the information. Newer VHF radios have a weather alert feature. If this feature is activated and a warning is broadcast, the radio will automatically tune in the appropriate channel.
If you are watching a TV weather report and forecast, it is just as important to take notes, dividing your attention between the picture on the screen and your notepad.
In most instances, you will be able to get a forecast for your local waters. If you can’t, try to get a forecast for the region upwind (usually westward) of you, and apply your knowledge of weather movement to make a forecast for your own area.
Keep your notes on weather forecasts received, and compare them with actual conditions later. You may be able to detect a pattern of error in the forecasts, such as weather frequently arriving 12 hours or so later than is forecast—apparently some forecasters would rather have you prepared a bit early for the arrival of a storm than to be caught short.
Figure 24-22 Although the National Weather Service has discontinued the use of the official systems of flags and lights shown here, the Coast Guard operates a Coastal Warning Display (Storm Flag) program at selected Coast Guard boat stations.
Weather Forecast Terms
The National Weather Service has an ascending series of alerting messages—advisories, watches, and warnings—for boaters, keyed to increasingly hazardous weather and sea conditions.
While the National Weather Service retired its Coastal Warning Display (Storm Flag) Program some time ago, the Coast Guard continues to display coastal warning signals at selected Coast Guard boat stations. The signals will inform and warn of conditions including small craft advisories, gale warnings, storm warnings, and hurricane warnings; see Figure 24-22. Signals may also be displayed at yacht clubs and marinas. In some areas, the Small Craft Advisory pennant is flown on law-enforcement patrol craft when that condition is established.
The Weather Service emphasizes that visual storm warnings are only supplementary to the written advisories and warnings given prompt and wide distribution by radio, television, and other media. Important details of the forecasts and warnings with regard to the time, intensity, duration, and direction of storms cannot be given satisfactorily through visual signals alone.
Use of Small-Craft Advisories
The term SMALL CRAFT ADVISORY needs some explanation. Although there is now no Weather Service definition of “small craft,” you can infer that this refers to small boats, yachts, tugs, barges with little freeboard, or any other low-powered craft. However, much depends on the skill of the mariner. It is the responsibility of all skippers to decide if their seamanship skills and the seaworthiness of their craft are capable of coping with expected wind and sea conditions. Each person has to make the decision to go or stay.
A small-craft advisory does not distinguish between the expectation of a general all-day blow of 25 knots or so, or, for example, a forecast of isolated, late-afternoon thunderstorms in which winds dangerous to small craft will be localized and of short duration. It is up to you to deduce from your own observations, supplemented by any information you can obtain, which type of situation the advisory applies to and to plan your day accordingly.
There is also a lower-level advisory, SMALL CRAFT SHOULD EXERCISE CAUTION, for situations posing lesser hazards.
Hurricane Watches & Warnings
Official information is issued by hurricane centers describing all tropical cyclone advisories, watches, and warnings in effect. These bulletins include tropical cyclone locations, intensity, movement, and special precautions that should be taken. ADVISORIES describe tropical cyclones and subtropical cyclones prior to the issuance of watches and warnings. When a hurricane threatens, an NWS watch or warning may result. A HURRICANE WATCH is an announcement for specific areas that a hurricane or incipient hurricane condition poses a threat to coastal areas—generally within 36 hours. A HURRICANE WARNING informs the public that sustained winds of 64 knots (74 mph) or higher associated with a hurricane are expected in a specific coastal area within 24 hours. This warning can remain in effect when dangerously high water or a combination of dangerously high water and exceptionally high waves continues, even though winds may be less than hurricane force.
Marine Weather Services Charts
The National Weather Service formerly published a series of 15 MARINE WEATHER SERVICES CHARTS covering the coastal waters of all 50 United States, Puerto Rico and the U.S. Virgin Islands, Guam and the northern Mariana Islands, and the Great Lakes. These charts, of which Figure 24-23 is an example, contain detailed information regarding the times of weather broadcasts from commercial stations, the locations of NWS continuous FM broadcasts and their frequencies, and much other information useful to boaters. Both sides of each of the charts could be downloaded in JPG or PDF format at www.weather.gov/om/marine/pub.htm. Due to budget constraints, however, these charts were no longer being updated as of mid-2012, and as of 2017 only the Alaskan Waters chart was available.
Figure 24-23 Marine Weather Service charts show the location, call signs, and frequencies of the VHF continuous weather broadcast stations in the charted area. Other information of general interest is also shown. As of 2017, the Alaskan Waters chart could be downloaded at www.weather.gov/om/marine/pub.htm,but other regional charts were no longer being updated due to budget constraints.
Boaters will find much interesting weather information on the various Pilot Charts of the North Atlantic and North Pacific oceans issued by the National Geospatial-Intelligence Agency (NGA). The Pilot Charts show average monthly wind and weather conditions over the oceans and in addition contain a vast amount of supplemental data on subjects closely allied to weather. They are a valuable source of information for skippers of all sizes of vessels on ocean passages;.
Each volume of the NOAA also contains information on climate conditions and weather broadcasts within its coverage area.
Weather Maps at Sea
Up-to-date weather maps are available when offshore with the use of facsimile radio (RADIOFAX) transmissions. The transmitted information can be automatically received and processed by a special-purpose FAX receiver for display on the vessel’s multifunction chartplotter; see Figure 24-24. The vessel’s SSB receiver can also be used to receive WEFAX signals for decoding and viewing on a general purpose computer equipped with the necessary software. Available charts include surface analysis, prognosis (forecasts for various periods), upper-air winds, wave analysis, seawater temperature, satellite photographs, and others.
Current and forecast weather information in text and GRIB file format can be downloaded from a LEO satellite via the Skymate system. Forecast and nowcast weather, including images from the weather radar network, can be received directly from geostationary satellites using services such as Sirius Weather, when within their coverage footprint.
Figure 24-24 Up-to-date weather maps can be received at sea via a weather fax receiver.
Weather Information from the Internet
The Internet has become and will continue to be a most important source for weather information of all types. The NWS implementation of graphic rather than word forecasts will continue to revolutionize the detail available from marine products. The place to start is the homepage of the National Weather Service at www.weather.gov. From here you can get almost any weather information you need—current conditions; local and marine forecasts, including offshore areas; weather maps; radar scans; satellite images; past weather and climate data; Marine Weather Service Charts, and many other items.
Weather Underground (www.wunderground.com) and the Weather Channel (www.weather.com.) provide a wide range of weather products of use to the mariner.
USING LOCAL WEATHER SIGNS
While weather forecasts are helpful, and you should always take care to get them, they are not completely dependable and they are normally made for a relatively large area. To supplement the official forecast, look around your boat frequently. Notice both the current weather and changes over the last hour or so. And finally, you should know how to interpret the weather signs that you see in clouds, winds, and pressure and temperature changes. If you are not in home waters, local advice may assist you with weather just as it does with navigation.
Cloud Observations
First, identify the cloud form (or forms) observed in the sky, and note whether they are increasing or decreasing in amount and whether they are lowering or lifting. In general, thickening and lowering of a cloud layer, be it a layer of cirrostratus, altocumulus, or altostratus, are signs of approaching wet weather. And when a layer of clouds shows signs of evaporation, that is, when holes or openings begin to appear in a layer of altostratus, or the elements of an altocumulus layer are frayed and indistinct at the edges, we have an indication of improving weather or, at least, delay of any development of foul weather.
Weather Proverbs
With all of modern weather forecasting’s scientific instruments, high-speed computers, and highly trained professionals, don’t overlook the guidance inherent in traditional weather proverbs. Their origins have been lost in the passage of time, and their originators did not know why they were true, but their survival over centuries attests to their general validity. Use them, with caution, and adapt them to local waters.
Finally, note the sequence of cloud forms during the past few hours. Cirrus clouds are frequently the advance agents of an approaching extra-tropical cyclone, especially if they are followed by a layer of cirrostratus. In this case, the problem is to forecast the track the low-pressure center will take to one side or the other of your location, the nearness of approach of the center, and the intensity of the low.
As the clouds thicken steadily from cirrus to cirrostratus and then to altostratus, you should expect further development to nimbostratus with its rain or snow. There are usually contrary indications if such is not to be the case or if the precipitation will arrive late, amount to little, end soon, or will come in two brief periods separated by several hours of mild, more or less sunny weather.
If the northern horizon remains clear until a layer of altostratus clouds overspreads most of the sky, the low-pressure center will probably be passing to the south without bringing precipitation. If the northern horizon is slow in clouding up but becomes covered by the time the cloud sheet is principally altostratus, there will probably be some precipitation but not much. If the cloud sheet, after increasing to altostratus, breaks up into altocumulus and the sky above is seen to have lost most of its covering of cirrostratus, the lowpressure area is either weakening or passing on to the north.
You can often detect the approach of a line of thunderstorms an hour or more in advance by observing the thin white arch of the cirrus border to the anvil top of the approaching cumulonimbus cloud. The atmosphere ahead of a squall line is often so hazy that only this whitish arch will reveal the presence of a moderately distant cumulonimbus, as the shadowed air under the dense anvil will be invisible behind the sunlit hazy blue air near the observer. Thus, this part of the sky will appear to be clear and will resemble the blue sky above the cirrus arch.
Mackerel skies and mares’ tails
Make tall ships carry low sails.
If high-flying cirrus clouds are few in the sky and resemble wisps in a mare’s tail in the wind, this is a sign of fair weather. Only when the sky becomes heavy with cirrus, or with mackerel, clouds—cirrocumulus that resemble wave-rippled sand on a beach—can you expect a storm. There is an exception to the mare’s tail proverb, however. If cirrus clouds form as mare’s tails with the hairs pointing upward or downward, the probability is for rain, even though the clouds may be scattered.
Rainbow to windward,
foul fall the day;
Rainbow to leeward, rains run away.
This is another old seafarer’s saying about rainbows, but it is certainly worth remembering because it is almost infallibly true. If a rainbow is behind or with the direction of the prevailing wind, then you can expect its curtain of moisture to reach you. But if the rainbow appears to the lee of the wind, then you know rain has already passed and the gray line of showers is receding, moving away from you.
Wind Observations
Sailboat skippers are familiar with the wind direction indicators that can be mounted at the masthead, and these are now often seen on medium-size and larger powerboats. Because a boat anchored or underway can head in any direction all around the compass, a few mental calculations are necessary before we can use this indicator, a yarn, an owner’s flag, or club burgee to determine the true direction of the wind. At anchor, the fly will give us the bearing of the wind relative to the boat’s bow; this must be converted to the true bearing of the wind itself. We can use our compass to obtain these values, provided we know its deviation on our heading and know the variation for our anchorage. In addition, we need to remember that wind direction is always stated as the true direction from which, not toward which, the wind is blowing.
For measuring wind strength we need something else. This is an ANEMOMETER; the principal parts of the instrument are illustrated in Figure 24-25, upper. The anemometer is essentially a speedometer. It consists of a rotor with hemispherical or conical cups attached to the ends of the spokes. It is designed for mounting at the masthead, where the wind is caught by the cups and causes them to turn at a speed proportional to wind speed. Indications of the rotor’s speed are electrically transmitted to an indicator mounted at the helm or in the cabin on your boat. In the type of instrument illustrated in Figure 24-25, lower, the indicator is direct reading, with its scale marked both in knots (0 to 50) and Beaufort force numbers (0 to 12); see below. Battery drain (0.09A) is insignificant. A 60-foot (18-m) cable is supplied to connect the rotor aloft with the illuminated indicator below. At anchor, the reading is the actual or true wind velocity at masthead height.
Models are available that have a digital display and can “remember” the maximum wind velocity since the last time that it was reset. Such instruments can also measure temperature and combine that data with wind speed to display a value for wind chill.
Figure 24-25 An anemometer rotor is shown above on this sailboat’s masthead fixture. It is connected by a cable to an indicator, usually mounted in the cockpit or at the navigator’s position. The display unit shown here, lower, also receives GPS course and speed input and is therefore able to convert apparent wind speed to true wind on demand. The needle shows the apparent wind angle.
True & Apparent Wind
So far we have been considering the determination of the direction and speed of the wind while at anchor. Underway, the determination is more difficult. But the Oceanographic Office of the Navy has worked out the problem, so we may as well use their results. Table 24-2 is based on wind tables published in Bowditch. Using our true course and speed, we can determine from this table the approximate TRUE WIND DIRECTION and SPEED, provided we know the apparent direction and speed. Our wind indicator or owner’s flag will give the apparent direction and our anemometer will give us the apparent strength.
Red sky at morning
Sailors take warning;
Red sky at night
Sailor’s delight.
There is a simple explanation for this old and quite reliable proverb. A red sunset results from viewing the sun through dusty particles in the air, the nuclei necessary for the formation of rain. This air probably would reach an observer the following day. Since weather tends to flow west to east in most places, if tomorrow’s weather appears to the west as a line of wetness, the sun shining through the mass appears as a yellow or grayish orb. On the other hand if the weather lying to the west is dry, the sun will show at its reddish.
Red sky in the morning is caused by the rising, eastern sun lighting up the advance guard of high cirrus and cirrostratus, which will be followed later on by the lowering, frontal clouds. Red sky at night—a red-tinted sunset—often derives from the sky clearing at the western horizon, with the clouds overhead likely to pass before the night is done.
Rainbow in morning
Sailors take warning;
Rainbow toward night
Sailor’s delight.
This is also quite an old weather jingle, and a little reasoning, particularly in view of the explanation about the “red sky at morning” saying, would tell you it is true.
As noted, storm centers usually move from the west. Thus, a morning rainbow would have to be viewed from its position already in the west with the sun shining on it from the east. (A rainbow is always seen in a direction opposite to the sun.) The storm would move in your direction, and you could confidently expect rain. A late-afternoon rainbow, however, viewed toward the east, would tell you that the storm has already passed.
The Beaufort Scale
Another method is to estimate the strength of the wind in terms of the BEAUFORT SCALE of wind force. Visually we can observe the sea condition and note the Beaufort number to which it corresponds; see Table 24-3. The range of wind speed represented by each force number on the Beaufort scale is shown in knots, statute miles per hour, and kilometers per hour in the table. Wind direction can also be judged by observing the direction from which the smallest ripples are coming since these ripples always run with the wind, responding instantly to changes in wind direction; see Figure 24-26.
You don’t need to determine wind direction and strength with great precision, but reasonable estimates are helpful in preparing your own local forecasts. Estimating direction within 15 degrees is sufficient and within one Beaufort number is enough for strength. By observing wind direction and speed regularly and making a record of them, you can obtain clues concerning potential weather developments as the wind shifts direction and changes its strength.
Figure 24-26 Photographs taken from a National Weather Service NOAA publication show wind forces and their effect on open waters. Beaufort Force 1 (calm air) is shown at upper left. Force 3 (gentle breeze) is at upper right. Force 5 (fresh breeze) appears at center left. Force 8 (gale) is at center right. Force 10 (storm) is shown at lower left. Finally, at lower right is Force 12 (hurricane); note that in hurricane-force winds the surface of the sea may be completely obscured by driving foam.
Wind Force & Its Effect on the Sea
Boaters frequently use the Beaufort scale to log wind speed and the condition of the sea, but too often the description of sea conditions relies on verbal statements like “moderate waves,” “large waves,” and “moderately high waves” (see Table 24-3, column for “Effects Observed on Water”) leaving the boaters pretty much “at sea” in trying to visualize exactly what these terms imply.
Table 24-2 This table will allow you to find the true force and direction of the wind from its apparent force and direction on a boat underway.
Table 24-3 This table is based roughly on a scale for estimating wind speeds developed in 1805 by Admiral Sir Francis Beaufort of the British Royal Navy. The original Beaufort Scale was based on the effect of various wind speeds on the amount of sail that a full-rigged ship could carry. It has since been modified and modernized.
THE BUYS-BALLOT’S LAW
Christophorus Buys-Ballot, a Dutch meteorologist, used the direction of the swirl of the wind in a low-pressure cell to find the center of a storm. In 1858, he formulated a law stating that winds are perpendicular to the lines of barometric slope. Two years later, he put his observations into a paper entitled Some Rules for Predicting Weather Changes in the Netherlands. In that paper was his well-known rule, “with your back to the wind, low pressure is to the left, higher pressure to the right.” (In the southern hemisphere, you would face the wind to find the low-pressure center with your left hand.) The application of this law has enabled many sailors to head for calmer waters.
The British Meteorological Office has solved this problem by issuing a State of Sea card (M.O. 688A) with photographs to accompany each of the descriptions of 13 wind forces of the Beaufort scale. Thus an observer has a guide in estimating wind strength (in knots) when making weather reports or in logging sea conditions. Fetch, depth of water, swell, heavy rain, current, and the lag effect between the wind picking up and the sea increasing may also affect the appearance of the sea. Range of wind speed and the mean wind speed are given for each force. By special permission, we reproduce six of these photographs (Forces 1, 3, 5, 8, 10, 12); refer to Figure 24-26. Forces 0, 2, 4, 6, 7, 9, and 11, though not illustrated, may be estimated in relation to those above and below them in the scale.
Pressure Observations
Another weather instrument you should be familiar with is the ANEROID BAROMETER; see Figure 24-27. The one illustrated has several interesting features.
First, there is the pressure scale. You probably are accustomed to thinking of barometric pressure in terms of INCHES OF MERCURY, so the scale is graduated in these units. Weather reports and forecasts now use pressures shown in MILLIBARS, so another scale graduated in millibars helps. You thus would not have to worry about conversions between units.
The “standard atmospheric pressure” of 29.92 inches of mercury is equal to 1013.2 millibars; 1 inch equals 33.86 millibars, or 1 millibar equals 0.03 inches of mercury.
Second, it is a rugged instrument and it has a high order of accuracy. Third, it has a reference hand for keeping track of changes in pressure. The words “Fair-Change-Rain,” in themselves, when they appear on the face of an aneroid barometer, are decorative. It is not the actual barometric pressure that is important in forecasting; it is the direction and rate of change in pressure.
How a Barometer Is Used
A good barometer is a helpful instrument provided you read it at regular intervals and keep a record of the readings and provided you remember that there is much more to weather than barometric pressure alone.
An individual reading of the barometer tells you only the pressure being exerted by the atmosphere on the earth’s surface at a particular point of observation at that time. But suppose you have logged pressure readings at regular intervals as follows:
Time | Pressure | Change |
0700 | 30.02 | __ |
0800 | 30.00 | -0.02 |
0900 | 29.97 | -0.03 |
1000 | 29.93 | -0.04 |
1100 | 29.88 | -0.05 |
1200 | 29.82 | -0.06 |
The pressure is falling, and it is falling at an increasing rate. Trouble is brewing. A fall of 0.02 inch per hour is a low rate of fall; consequently, this figure would not be particularly disturbing. But a fall of 0.05 inch per hour is a rather high rate.
Next, there is a normal daily change in pressure. The pressure is usually at its daily maximums about 1000 and 2200 and at its daily minimums about 0400 and 1600. The variation between minimum and maximum may be as much as 0.05 inch change in these six-hour intervals (about 0.01 inch change per hour). Thus, when the pressure normally would increase about 0.03 inch (0700 to 1000) our pressure actually must have fallen 0.09 inch.
Suppose, now, that at about 1200 you also observed that the wind was blowing from the NE with increasing force and that the barometer continued to fall at a high rate. A severe northeast gale is probably on its way. On the other hand, given the same barometer reading of about 29.80, rising rapidly with the wind going to west, you could expect improving weather. Quite a difference!
Figure 24-27 The barometer shown here has scales both in inches of mercury and in millibars. Markings such as “Rain,” “Change,” or “Fair” are traditional but are of little practical value.
Barometric Changes & Wind Velocity
Let us now relate barometric changes to wind velocity. First, it is generally true that a rapidly falling barometer forecasts the development of strong winds. This is so because a falling barometer indicates the approach or development of a low, and the pressure gradient is usually steep in the neighborhood of a low-pressure center. On the other hand, a rising barometer is associated with the prospect of lighter winds to come. This is true because a rising barometer indicates the approach or development of a high, and the pressure gradient is characteristically smaller in the neighborhood of a high-pressure center.
The barometer does not necessarily fall before or during a strong breeze. The wind often blows hard without any appreciable accompanying change in the barometer. This means that a steep pressure gradient exists (isobars close together, as seen on the weather map), but that the well-developed high or low associated with the steep pressure gradient is practically stationary. In this case the wind may be expected to blow hard for some time; any slackening or change will take place gradually.
It not infrequently happens that the barometer falls quite rapidly, yet the wind remains comparatively light. If you remember the relation between wind velocity and pressure gradient, you can conclude that the gradient must be comparatively small (isobars relatively far apart). The rapid fall of the barometer must be accounted for, then, in either one or two ways. Either a low with a weak pressure gradient on its forward side is approaching rapidly, or there is a rapid decrease of pressure taking place over the surrounding area, or both. In such a situation, the pressure gradient at the rear of the low is often steep, and in that case, strong winds will set in as soon as the barometer commences to rise. (It will rise rapidly under these circumstances.) The fact that the barometer is now rising, however, indicates that decreasing winds may be expected soon.
ELECTRONIC BAROMETERS
In this age of high-tech electronic devices on boats, there are now ELECTRONIC BAROMETERS. These can provide an easily read digital display of atmospheric pressure plus an expanded history of changes that occurred at regular intervals over the previous hours. Audible alarms can be set to sound at preset low pressures or for falling pressure at a selected rate. Time and temperature can also be continuously displayed. These units are powered by an internal battery and may be placed anywhere convenient.
The Barometer & Wind Shifts
Nearly all extra-tropical cyclones display an asymmetrical distribution of pressure. The pressure gradients are seldom the same in the front and rear of an extra-tropical cyclone. During the approach of a low, the barometer alone gives no clue as to how much the wind will shift and what velocity it will have after the passage of the lowpressure system. This is particularly applicable to situations in which the wind blows from a southerly direction while the barometer is falling. The cessation of the fall of the barometer will coincide with a veering (gradual or sudden) shift of the wind to a more westerly direction. Unfortunately, if you have no information other than the variations in atmospheric pressure indicated by your barometer, you cannot foretell the exact features of the change.
In using barometric indications for local forecasting, remember that weather changes are influenced by the characteristics of the earth’s surface in your locality. Check all rules against experience in your own cruising waters before you place full confidence in them.
A backing wind says
storms are nigh;
But a veering wind will clear the sky.
This folklore observation generally refers to storms running in a southerly direction.
Appearance of the
Moon Proverbs
The shape and color of the moon as indicators of coming weather changes have long been a subject of controversy—mostly among those meteorological experts who declare that the moon has no appreciable control over the weather beyond a very small tidal effect on the atmosphere. But for now, let’s be content with the observation that as far as weather portents are concerned, the moon is one of the most visible and absolutely reliable signs of weather change. It is not the moon’s influence that makes the following sayings ring with truth, it is other atmospheric conditions that influence the moon’s appearance.
Temperature Observations
Thermometer readings will not give as much information for weather predicting as data from other instruments, but they are not without some value.
Cold air carried down from a thunderstorm cloud with the rain may be felt as much as three miles in advance of the storm itself. Thus, a warning is given and the approach of the storm is confirmed.
Sharp horns on the moon
threaten high winds.
When you can clearly see the sharp horns or ends of a crescent moon with your naked eye, it means there are high-speed winds aloft that are sweeping away cloud forms. Inasmuch as these high winds always descend to earth, you can predict a windy day following.
When a halo rings the moon or sun
The rain will come upon the run.
Halos are excellent atmospheric signs of rain. Halos around the moon after a pale sun confirm the advent of rain, for you are viewing the moon through the ice crystals of high cirriform clouds. When the whole sky is covered with these cloud forms, a warm front is approaching, bringing a long, soft rain.
GENERAL BAROMETER RULES
• Foul weather is usually forecast by a falling barometer with winds from the east quadrant.
• Except after a wintertime cold frontal passage in a relatively warm marine area, clearing and fair weather is usually forecast by winds shifting to west quadrants from a rising barometer.
• When the wind sets in from points between south and southeast and the barometer falls steadily, a storm is approaching from the west or northwest, and its center will pass near or north of the observer within 12 or 24 hours, with the wind veering to northwest by way of south and southwest.
• When the wind sets in from points between east and northeast and the barometer starts to fall steadily, a storm is approaching from the south or southwest, and its center will pass near or to the south of the observer within 12 or 24 hours, with the wind backing to northwest by way of north.
• The rapidity of a storm’s approach and the storm’s likely intensity will be indicated by the rate and amount of fall in the barometer.
• A falling barometer and a rising thermometer often forecast precipitation.
• Barometer and thermometer rising together often forecast fine weather.
• A slowly rising barometer forecasts settled weather.
• A steady, slow fall of pressure indicates forthcoming unsettled or wet weather.
Judging the Likelihood of Fog
In order to judge the likelihood of fog formation, you can periodically measure the air temperature and dew-point temperature and see if the SPREAD (difference) between them is getting smaller.
A graph derived from a series of air and dewpoint temperatures is shown in
Figure 24-28. By recording these temperatures and plotting their spread over a period of several hours, you will have a basis for forecasting the time at which you are likely to be fog-bound. This entire curve represents actual data, but it could just as easily have been completed late Friday evening by extrapolating forward from the observations recorded to that point. If an error were made in this extrapolation, it would probably indicate that the fog would form at an earlier hour. This is on the safe side; you would be secure in your anchorage sometime before the 59th minute of the 11th hour.
Figure 24-28 Shown here is a plot of a series of air and dew point temperatures at hourly intervals based on the values in the table, inset. Indications of the density of fog at various times are also shown.
Note that while the average decrease in spread is about 1.5°F per hour, the decrease is at a much greater rate in the earlier hours of the day. So long as you do not make unreasonable allowances for a slowing up in the rate of change of the spread, this also will help keep you on the safe side.
When boat horns sound hollow,
Rain will surely follow.
Anyone who has spent any time around boats knows the truth of this timehonored prophecy. Nor do you have to be sitting on a piling in a marina to notice the unusual sharpness of sounds on certain days—the more penetrating sound of a bell ringing or voices that carry longer distances are signs of the acoustical clarity that results from bad weather lowering the cloud ceiling toward the earth. The tonal quality of sound is improved because the cloud layer bounces the sounds back, the way the walls of a canyon echo a cry. When the cloud barrier lifts, the same clouds dissipate in space.
A halo around the sun indicates the
approach of a storm
within three days, from the side
which is the most brilliant.
Halos predict a storm at not great
distance, and the open
side of the halo tells the quarter from
which it may be expected.
These two sayings at first reading may seem contradictory, but they are more explicit in their forecasting than the simple “halo rings the moon” saying. As cirrus and cirrostratus fronts push across the sky in the region of the moon or sun, the halo first appears and subsequently becomes brightest in that part of the arc from which a low-pressure system is approaching. Later, the halo becomes complete and the light is uniform throughout. As the storm advances, altostratus clouds arrive and obliterate the original, and for a time, the brightest part of the halo—that is, the side nearest the oncoming storm. Both sayings are useful, but they refer to different times in the life of the halo.
It is also true that when halos are double or triple, they signify that cirrostratus clouds are relatively thick, such as would be the case in a deep and well-developed storm. Broken halos indicate a much disturbed state in the upper atmosphere, with rain close at hand.
Now, to put any confusion to rest about the forecast persistence of rain by the appearance of sun and moon halos, the U.S. Weather Service has verified through repeated observations that sun halos will be followed by rain about 75 percent of the time. Halos around the moon have a rain forecasting accuracy of about 65 percent.
Determining the Dew Point
You can determine the dew point by means of a simple-to-operate, inexpensive device known as a SLING PSYCHROMETER; see Figure 24-29. A sling psychrometer consists of two thermometers mounted in a single holder with a handle that permits it to be whirled overhead. One thermometer, known as the DRY BULB, has its bulb of mercury exposed directly to the air and thus shows the actual temperature of the air. The other thermometer, the WET BULB, has its bulb covered with a piece of gauze; soak this gauze in fresh water so that the bulb is moistened. If the air is not saturated with water vapor, evaporation then takes place from the wet-bulb thermometer as it is whirled and, since the process of evaporation requires the expenditure of heat, the wet bulb is cooled. Continue whirling until it can be lowered no longer. The reduced temperature shown by the wet-bulb thermometer, the WET-BULB TEMPERATURE, thus represents the lowest temperature to which the air can be cooled by evaporating water into it.
Figure 24-29 This pockettype sling psychrometer has two 5-inch tubes, etched with divisions reading from 20 degrees to 120 degrees Fahrenheit.
When you whirl the psychrometer you create a draft around it, thereby increasing the efficiency of the evaporation process and making the wet bulb more reliable than it would be with little or no air movement past it—hence the psychrometer’s design for whirling.
From the wet-bulb and dry-bulb temperatures, you can determine the dew point by referring to a suitable table. As you are far more interested in knowing the spread between air temperature and dew point, however, use Table 24-4,.
If the air is already saturated with water vapor, no water can evaporate from the gauze and both thermometers will show the same value. The spread between air temperature and dew point is zero. But if the air is not already saturated with water vapor, subtract the wet-bulb temperature from the dry-bulb temperature. With this difference and the dry-bulb (the air) temperature, consult Table 24-4 and find the corresponding spread between the air temperature and the dewpoint temperature. This is the figure you want.
If, in the late afternoon or early evening, the spread between the air temperature and dew point is less than approximately 6°F, and the air temperature is falling, you will probably encounter fog or greatly restricted visibility in a few hours. These critical values are emphasized by the heavy line below them in Table 24-4.
Incidentally, should you ever want to know the dew-point temperature itself, all you need do is to subtract the spread figure given in the table from the temperature shown by the dry-bulb thermometer. The formula used for converting Fahrenheit temperature to Celsius temperature is: (°F-32)x0.56=°C.
The Weather Log
Although the latest Weather Service forecasts are readily available via various media, it is often helpful to record your own cloud and weather observations. You can then check the reliability of the latest prediction. Occasionally, the professional forecaster misjudges the future rate of travel of the weather pattern, which may move faster or slower than anticipated. Or a new, unforeseen development in the pattern may occur.
Sound traveling far and wide
A stormy day does like betide.
This is another (English) version of the saying about “sound” and bad weather; this one suggests that you actually can hear bad weather approaching, say, when a faraway train whistle is audible when normally it would be faint. The reason the sound carries farther is that the whistle was blown under a lowering cloud ceiling whose extending barrier may not have reached your position yet.
A form of WEATHER LOG that is suitable for use on recreational boats is shown in Figure 24-30, left. This is the front sheet; Figure 24-30, right, shows the reverse side. The first weather items recorded are based on a reading of the latest weather map (if available) and a summary of any radio reports received. Then use the reverse side to jot down your local observations. Sufficient columns are available to permit the entry of these data six times during one 24-hour day, at four-hour intervals. Entries may be made using standard weather code symbols or any other method you choose, provided you use the same system consistently.
Figure 24-30 A weather log of this general format, left, can be used to record information developed from weather maps, or received by radio, and the forecast developed from this data. The reverse side, right, is used for local weather observations. By recording this information and keeping the record, it is possible to develop considerable skill in making forecasts.
From these records you can estimate how and to what extent the actual weather during the next few hours might differ from those predicted in the official forecast.
Using readily available communications technology, a boater can forward significant weather observations to the Weather Service at www.nws.noaa.gov. This is one way the boating public has of improving forecasts and warnings in the marine environment.
Table 24-4 This table will allow you to use the air temperature as shown by the dry-bulb thermometer of a sling psychrometer and the difference between that reading and the wet-bulb reading to determine dew-point spread. All figures are in degrees Fahrenheit at 30 inches barometric pressure.
Lightning from the west or
northwest will reach you,
Lightning from the south or southeast
will pass you by.
This is a true saying, if you live in the North Temperate Zone. Lightning comes hand in hand with storm clouds, and thunderheads always loom over the horizon from the west or northwest and usually move east. So lightning anywhere from the south or southeast will pass you by.