Clouds

Clouds are formed when air cools to its dew point or becomes saturated, at which point the air cannot hold any more water vapor, and water droplets condense. There are many different conditions under which this occurs, producing a variety of cloud types. The force with which the air rises, the topography (features of the surface), and the temperature at the surface and at various altitudes are among the factors that influence whether a cloud will be small or large, thick or thin, flat or bumpy, and whether it will produce precipitation.

Cloud classification systems

The first scientific method of classifying clouds was devised in 1803 by English naturalist and pharmacist Luke Howard (1772–1864). In an article entitled "On the Modifications of Clouds," Howard assigned Latin names to four different cloud categories, based on appearance: cumuliform ("piled") for puffy, heaped-up clouds; cirriform ("hairlike") for thin, wispy, feathery swirls of clouds; stratiform ("layered") for continuous, flat sheets or layers of clouds; and nimbus ("cloud") for dark rain clouds. He then used combinations of these names to describe other clouds. For instance, nimbostratus is a rain-producing, layered cloud, and stratocumulus is a continuous cloud sheet punctuated by bumps.

This system of cloud classification was revised in 1874, at the first meeting of the International Meteorological Congress. There it was decided to use Howard's cloud names as a starting point for a classification system that placed clouds into ten categories, based on their height in the sky as well as their appearance. In 1896, the Congress published the International Cloud Atlas, the general outline for the system still in use today.

WORDS TO KNOW

calvus:

"bald"; upper part of a cloud is losing its rounded, cauliflower-like outline and becoming diffuse and fibrous. Applies to Cu, Cb.

capillatus:

"having hair"; cloud with cirriform, streaky structure on its upper edges. Applies to Cb.

castellanus:

"castlelike"; vertical extensions. Applies to Cc, Ac, Sc, Ci.

congestus:

"congested"; upper parts piled up and sharply defined; resembles a head of cauliflower. Applies to Cu.

fibratus:

"fibrous"; hairlike strands with no hooks or curls at the end. Applies to Ci, Cs.

floccus:

"flock of wool"; small tufts with ragged undersides. Applies to Cu, Ci, Cc, Ac.

fractus:

"fractured"; broken up, ragged edges. Applies to St, Ci, Cu (it is used as a prefix in the case of Cu "fractocumulus").

humilis:

"humble, lowly"; small, flattened appearance. Applies to Cu.

incus:

"anvil," or fan-shaped; spreading, smooth or fibrous mass at the top of a cloud. Applies to Cb.

intortis:

"intertwined"; entangled, fibrous strands. Applies to Ci.

lenticularis:

"lens-shaped"; elongated, or almond-shaped with well-defined outlines. Applies to Cs, Cc, Ac, Sc.

mammatus:

"breast, udder"; pouches of water droplets that hang from the underside of a cloud. Applies to Cb, Ci, Cc, Ac, As, Sc.

mediocris:

"mediocre"; moderate vertical development with lumpy tops. Applies to Cu.

nebulosus:

"nebulous"; thin, hazy veil. Applies to Cs, As, St.

pileus:

"felt cap"; small cap- or hood-shaped formation perched above or attached to the top of a cloud. Applies to Cu, Cb.

spissatus:

"tightly packed"; icy formations at the top of a vertical cloud that are dense enough to block out the Sun. Applies to Ci.

stratiformis:

"covering, blanket"; thick layer. Applies to Ac, Sc, Cc.

translucidus:

"translucent"; transparent layer covering a large part of the sky, through which the Sun or Moon shines. Applies to St, As.

troposphere:

the lowest atmospheric layer, where clouds exist and virtually all weather occurs.

uncinus:

"hook-shaped"; fibers creating the pattern called "mare's tail." Applies to Ci.

undulatus:

"undulating"; wavelike formation within patches, layers, or sheets of clouds. Applies to Ac, As, Cc.

Low clouds are those with a base below 6,500 feet (2 kilometers); middle clouds are those with a base between 6,500 and 23,000 feet (2 and 7 kilometers); and high clouds are those with a base between 16,000 and 43,000 feet (5 kilometers and 13 kilometers). It's important to remember that, for classification purposes, it is the bottom, or base, of a cloud that determines its height.

You may notice that there is some overlap between the altitude of cloud bases in the middle- and high-cloud categories. The altitude at which clouds form depends on air temperature. On warm days and in warmer locations, medium and high clouds form at higher altitudes than they do on cold days.

This list of cloud-base altitudes pertains only to the middle latitudes, or temperate regions, which lie between 30° and 60° north and south of the equator. In the tropics, the bases of middle and high clouds form at higher altitudes than they do in temperate regions. In the polar regions these cloud bases form at the lower altitudes than they do in temperate regions.

There also exists a fourth group of clouds, the tops of which may extend to the edge of the troposphere, the lowest layer of the atmosphere. These clouds are called towering cumuliform clouds. Sometimes towering cumuli-form clouds are placed with other clouds in the low category, since their bases

form below 10,000 feet (3 kilometers). In most classification schemes, however, including the one in this book, cumuliform clouds are placed in a category of their own, called vertical clouds.

The ten basic categories of clouds, arranged by base height and appearance, are called genera. Each genus (singular form of "genera") is subdivided into species.

Low clouds

Low clouds are almost always composed of liquid water droplets. When the temperature drops below 23°F (?5°C), however, these clouds may also contain ice crystals. There are three genera of low-lying clouds: stratocumulus, stratus, and nimbostratus.

Stratocumulus (Sc)

Stratocumulus (pronounced stray-toe-KYOOM-yuh-luss) is a common type of cloud. As its name suggests, stratocumulus is a layered, puffy hybrid of a cloud. The puffiness is a result of warm air rising above the base of the cloud and condensing at higher altitudes. Stratocumulus ranges in color from white to dark gray, depending on its thickness.

Stratocumulus forms wide, shallow layers and may blanket the entire sky or may have breaks through which blue patches are visible. It may appear as a series of distinct, yet touching, rounded masses. Sometimes those masses appear in rows.

The presence of stratocumulus is an indicator of high levels of moisture in the lower levels of the troposphere. It is formed either when pockets of warm air rise to the dew point or when a warm air mass is pushed upward by an advancing front, the dividing line between two air masses. Usually, stratocumulus clouds do not produce precipitation. However, when these clouds become thick enough, they may bring light drizzle or snow.

Experiment: Make a cloud in a bottle

For this experiment you will need a large glass jar (like a pickle or mayonnaise jar), a metal baking tray full of ice cubes, a piece of black paper, and matches.

First, affix the black paper to the back of the outside of the jar, so your cloud will be easier to see. Next, fill the jar about one-quarter full with hot water (but not boiling water, because this can crack the glass). Then carefully light a match, hold it over the jar opening for a few seconds, and drop it into the jar. Quickly place the tray of ice on top of the jar and observe what happens inside the jar.

This experiment reproduces the circumstances under which clouds are formed. Namely, the surface is warmed, the lowest layer of air rises, and water changes from gas to a liquid as the air cools. This process is called condensation. The smoke from the match provides the condensation nuclei, which are small particles around which water droplets form.

Stratus (St)

The second type of low cloud, stratus (pronounced STRAY-tuss), is that gloomy, gray, featureless sheet of cloud that covers the entire sky. It is common worldwide and is noted for blanketing coastal

and mountainous areas for long periods of time. Stratus may produce drizzle or, if it's cold enough, light snow.

Stratus formation takes place at a lower altitude than any other cloud type. Stratus is most often a shallow layer of cloud, sometimes appearing nearly transparent. It may develop to a maximum thickness of 1,500 feet (450 meters). A stratus sheet typically covers an area of hundreds of square miles across.

Stratus clouds are usually formed by the rising of a large mass of moist air. In some cases, however, stratus is formed when a layer of fog (a cloud that forms near the ground), is warmed by the Sun, and then rises from the ground. Alternately, the rising fog may create a layer of stratus that is uneven and puffy, more accurately described as stratocumulus.

Nimbostratus (Ns)

Finally, there are wet-looking nimbostratus (pronounced nim-bo-STRAY-tuss) clouds. These clouds are similar to stratus clouds in that they form a gray layer that covers all or a large part of the sky. Nimbostratus clouds, however, are thicker and darker than stratus clouds. They are often jagged at the base, a result of being blown about by the wind. Nimbostratus clouds may fuse with stratocumulus clouds below or altostratus clouds above, making their upper and lower edges difficult to distinguish.

Nimbostratus often brings continuous, light to heavy precipitation that lasts more than twelve hours. This precipitation may evaporate and

produce a low-lying cloud or fog, further reducing visibility on the ground. The base of a nimbostratus cloud may form as high as 13,000 feet (4 kilometers) above ground. For this reason, nimbostratus is sometimes classified as part of the middle group of clouds.

Middle clouds

Middle clouds are those with bases that form about 6,500 feet (2 kilometers) to 23,000 feet (7 kilometers) above Earth's surface. The temperature of the air at this elevation is usually between 32°F (0°C) and ?13°F (?25°C). Thus, these clouds contain supercooled water, which remains in a liquid state below freezing point, or a combination of supercooled water and ice. There are two types of middle clouds: altocumulus and altostratus.

Altocumulus (Ac)

Altocumulus (pronounced all-toe-KYOOM-yuh-luss) clouds are puffy masses, the bases of which are higher than ordinary cumulus clouds. Altocumulus clouds often appear in parallel rows or waves, comprised of thousands of small clouds, and may be several layers thick. These clouds are noted for the picturesque patterns they form. They are produced by the lifting of warm air that often precedes an advancing cold front, the line behind which a cold air mass is advancing.

Within a single altocumulus cloud, one will find areas of light and dark gray and even white. The edges of each puffy mass are pronounced, indicating the presence of water droplets.

Altocumulus clouds are produced in a similar fashion to altostratus clouds, namely by the uplift of a large air mass and the resulting condensation of water droplets. The difference between the two processes has to do with air stability. When the atmosphere is unstable around the dew point, warm air continues to rise and condense. This leads to the formation of altocumulus. When the air is stable, altostratus is formed.

When warm air rises far above the base of altocumulus clouds, the puffy masses appear tall and are described as little castles. When little castles are present in the morning on a warm, humid day, it is a sign that afternoon thunderstorms are likely.

Altostratus (As)

The Latin prefix alto- means "high" and stratus means "sheetlike." Thus, altostratus (pronounced all-toe-STRAY-tuss) clouds are flat sheets, the base of which is higher than ordinary stratus clouds. Similar to stratus, this plain layer of cloud covers the entire sky.

Altostratus is a white, gray, or blue-gray uniform cloud sheet that may, like stratus, blanket an area as large as hundreds of square miles. It is generally thin enough that a dim outline of the Sun (called a "watery sun") or Moon can be seen through it. Sometimes, however, it is thick enough to entirely block our view of the Sun or Moon.

Unlike altostratus, nimbostratus, a darker gray, is so thick that it always hides the Sun. Also, nimbostratus looks textured whereas altostratus is more likely to look smooth. Altostratus can be told from stratus because stratus is the lower and darker of the two.

Altostratus clouds are produced when a large air mass rises, often pushed upward by an incoming front, and cools to the dew point. When a layer of altostratus is relatively thin, it does not generally yield precipitation. However, when it is thick enough, it will produce rain and snow over an extensive area. Altostratus clouds often precede an advancing storm system.

High clouds

High clouds are those with bases situated more than 20,000 feet (6 kilometers) above Earth's surface. They are formed when water condenses out of the air at high altitudes. In very cold weather in the middle latitudes they may form at altitudes as low as 16,000 feet (5 kilometers). The temperature of the air where high clouds form is generally below ?13°F (?25°C), and the moisture content is low. Thus, these clouds are composed mostly of ice crystals, with small amounts of liquid water. This gives them their thin, wispy appearance.

Cirrus (Ci)

The most common cirriform clouds are cirrus (pronounced SEER-us). These clouds are created by windblown ice crystals and are so

thin as to be nearly transparent. Cirrus clouds may resemble long streamers, feathery patches, strands of hair with a curl at the end, or a number of other distinctive shapes. The wispy appearance of some cirrus clouds have earned them the nickname "mares' tails."

Cirrus clouds may appear in small patches or extensive areas of the sky. In the former case, they are associated with fair weather. These clouds are carried across the sky from west to east by the prevailing winds, the winds blowing in the direction that is observed most often during a given time period. Sailors used to measure the speed of winds aloft, which blow in the middle and upper layers of the troposphere, by the appearance of cirrus clouds: the longer the streamers, the faster the wind.

When the cirrus cover grows thicker in the west and takes on a crisscross pattern, it means that warm air is advancing at high altitudes. This is the first sign of an approaching warm front. After that, one can expect to see the development of a thick layer of cirrostratus. Clouds will develop at progressively lower heights as the rain approaches.

Cirrostratus (Cs)

Cirrostratus (pronounced seer-oh-STRAY-tuss) clouds are a higher, thinner version of altostratus clouds. They cover all or part of the sky in a sheet thin enough that the Sun or Moon are clearly visible through them. When these clouds are present there is often a halo,

a ring of light, around the Sun or Moon. The halo is produced by the refraction, or bending, of light through the ice crystals within the clouds. In some cases, when cirrostratus clouds are nearly transparent, a halo is all that marks their presence. When these clouds are thicker, they appear as a milky white sheet across the sky.

Cirrostratus clouds are formed by a large-scale, gentle lifting of moist air to great heights. This rising air is a result of convergence, the flow of winds inward toward an area on the surface.

Cirrostratus is thinner and lighter in color than altostratus. Another way to tell the two cloud types apart is that only cirrostratus allows enough sunlight to pass through to create shadows on the ground.

Snow rarely falls from cirrostratus clouds, and when it does, it usually takes the form of virga, meaning it evaporates before it hits the ground. A layer of cirrostratus clouds that's growing thicker often represents the leading edge of a warm front. If a band of middle clouds shows up next, you can expect rain or snow in the next twelve to twenty-four hours.

Cirrocumulus (Cc)

The most uncommon type of cirriform clouds is cirrocumulus (pronounced seer-oh-KYOOM-yuh-luss). These are small, white, rounded, and puffy clouds. They may occur individually or in patterns resembling rippled waves or the scales of a fish (the latter is

termed "mackerel sky"). Cirrocumulus is noted for its distinctive patterns and for the beautiful shades of red and yellow it takes on during sunrises and sunsets.

Cirrocumulus clouds resemble altocumulus clouds but exist at higher altitudes. The two cloud types can be told apart because cirrocumulus clouds are even-colored, as opposed to altocumulus clouds, in which some areas are darker than others. Also, cirrocumulus clouds have smaller individual puffs than do altocumulus clouds.

Cirrocumulus clouds usually cover a small portion of the sky; only rarely do they cover the entire sky. They generally form thin layers and block very little sunlight. In fact, similar to the case of cirrostratus clouds, enough sunlight shines through cirrocumulus cloud for shadows to appear on the ground.

Cirrocumulus itself does not yield precipitation. However, if the cloud layer begins to thicken it may indicate that a front is on the way.

Vertical clouds

The clouds included in this category are the products of sudden, forceful uplifts of small pockets of warm air. To produce a vertical cloud, the air must be thrust upward at a speed of about 70 miles per hour (mph) or 113 kilometers per hour (kph). This can happen either by convection, the upward motion of a heated air mass, or by frontal uplift, in which the warm air mass rises due to an approaching cold air mass. In contrast, the uplift of air that produces other cloud types usually occurs at a rate of less than 1 mph (1.6 kph).

The base of a vertical cloud is usually between 3,600 and 6,600 feet (1 and 2 kilometers) above the ground. These clouds are found in all parts of the world except Antarctica, where surface temperatures are so cold that convection can not take place.

Within this category are cumuliform clouds, including cumulus and cumulonimbus (thunderstorm clouds). It's important to note, however, that altostratus and nimbostratus demonstrate varying degrees of vertical development, and for that reason are also sometimes considered vertical clouds.

Cumulus (Cu)

Cumulus (pronounced KYOOM-yuh-luss) clouds are quite easy to recognize. Formed on humid days, they look like white or light-gray cotton puffballs of various shapes set against the blue sky. They are typically about a half-mile wide, and their edges are clearly defined.

Cumulus can be distinguished from stratocumulus because cumulus clouds exist singly while stratocumulus clouds exist in groups or rows. While cumulus clouds have rounded tops, stratocumulus clouds have relatively flat tops.

Cumulus clouds are produced by convection, the rising of pockets of warm air. This occurs as the Sun warms the ground and the layer of the air above it. The warm air rises, forming a low pressure area. Surrounding air flows in and is also warmed. This process continues until a convection cell is formed. As the air rises, it expands and cools. If it cools below the dew point, clouds will form. Cumulus clouds usually begin forming in the morning and grow throughout the day. They reach their tallest point at the warmest time of day, which is generally midafternoon. In the evening, they begin to dissipate.

As long as the atmosphere is unstable (the ascending air parcel, a small portion of air with a consistent temperature, is warmer than the ambient air) at the height of the cloud base, the cumulus cloud will continue to develop vertically. The top of a cumulus cloud indicates the

limit of rising air. Cumulus clouds typically grow to only moderate heights and are associated with fair weather.

When the air is particularly unstable and strong convection occurs, a cumulus cloud can reach great heights. As it surges upward, it passes through the intermediate stage of cumulus mediocris, a medium-sized cumulus with a lumpy top, and then may become cumulus congestus, which is shaped like a head of cauliflower. During this stage, cumulus clouds grow wider and often run into one another, forming a line of towering rain clouds. If the vertical growth of a cumulus congestus continues, it will evolve into a cumulonimbus, or thunderstorm cloud.

Cumulonimbus (Cb)

While the dark base of a cumulonimbus (pronounced KYOOM-yuh-low-NIM-bus) can form as low as 1,000 feet (300 meters) above the ground, its top may reach 39,000 feet (12 kilometers), which is well into the upper reaches of the troposphere. In the tropics and subtropics, the top of the largest species of thunderstorm clouds, cumulonimbus incus, a fully developed cumulonimbus that reaches the top of the troposphere, can surge beyond the troposphere and into the stratosphere, the second-layer of the atmosphere.

A cumulonimbus cloud will keep growing taller as long as both convection and atmospheric instability persist. The atmosphere is considered unstable when the temperature of the surrounding air drops with

increasing altitude, at a faster-than-average rate. On average, air temperature drops 3.6°F for every 1,000 feet (6.5° per kilometer) one ascends.

If a cumulonimbus cloud extends into the stratosphere, it will encounter a reversal in the cooling trend: Temperature in the stratosphere rises with altitude. This change brings a halt to the cloud's vertical growth. If the updrafts, upward-blowing columns of air, continue within the cloud, it will grow outward. Ice crystals at its top will then be sheared off by the jet stream, the fastest upper-air winds, and fan outward into a wedge-shaped mass, forming cumulonimbus incus. This cloud is so-named because its top is similar in appearance to a blacksmith's anvil, the Latin name for which is incus. Cumulonimbus incus clouds may appear singly or in ominous-looking rows called squall lines.

Ice that shears off the top of this cloud's anvil may form layers of cirrus and cirrostratus clouds that cover an area hundreds of miles downwind. For this reason, cumulonimbus is also referred to as a cloud factory.

The lower portions of a cumulonimbus cloud contain liquid water, the middle portions contain both water and ice, and the top is made entirely of ice crystals. Therefore, one cloud can simultaneously produce different forms of precipitation, including rain, snow, and hail, in great quantities.

A cumulonimbus cloud is a giant storehouse of energy. Within it are powerful updrafts and downdrafts of wind, blowing at speeds greater than 55 mph (88 kph). Thunder, lightning, and tornadoes all may accompany storms produced by cumulonimbus clouds.

Variations in cloud base heights

The range of altitudes listed in the previous section for middle and high clouds applies only to the middle latitudes, or temperate regions. The bases of middle and high clouds form at highest elevations in tropical regions (30° south to 30° north) and lowest elevations in the polar regions (60° to 90°, north and south).

Specifically, the bases of middle clouds form from 6,500 to 13,000 feet (2 to 4 kilometers) in the polar regions; 6,500 feet to 23,000 feet (2 to 7 kilometers) in the temperate regions; and 6,500 to 26,000 feet (2 to 8 kilometers) in the tropical regions. The bases of high clouds form from 10,000 to 26,000 feet (3 to 8 kilometers) in the polar regions; 16,000 feet to 43,000 feet (5 to 13 kilometers) in the temperate regions; and 20,000 to 60,000 feet (6 to 18 kilometers) in the tropical regions. The base height of low clouds is unaffected by latitude.

These variations in cloud base height are due to temperature. Polar air is generally colder than temperate or tropical air at equal elevations throughout the troposphere. Thus, air cools to its dew point relatively close to the ground in the polar regions, whereas it must travel to greater heights before condensation occurs in the tropics. For instance, cirrus clouds, high clouds made of ice crystals, can form as low as 10,000 feet (3 kilometers) in polar regions, whereas they will form only at twice that height in the tropics.

The ranges of cloud base heights also show some variation within a given geographic region due to the season and time of day. Again, the cause of this variation is air temperature. In the winter, middle and high clouds form at lower heights than they do in summer. Cloud base height, on average, is highest in midafternoon when the air is warmest and lowest at night when the air is coldest.

Cloud species

To review, the ten basic groups, or genera, of clouds are stratocumulus, stratus, nimbostratus, altocumulus, altostratus, cirrus, cirrostratus, cirrocumulus, cumulus, and cumulonimbus. Within each genus (the singular of genera), there is one or more species. Species are highly defined types of clouds.

Much can be learned about a cloud by translating its species name from the Latin. For instance, lenticularis means "lens" and describes a lens-shaped cloud; fractus means "fractured" and describes a cloud with irregular or ragged edges; and pileus means "piled up" and refers to clouds with a caplike formation on top.

Some clouds species are subdivided into varieties. One way to name a variety is to list more than one species name after the genus name. An example of this is Cumulus congestus pileus, a cauliflower-shaped, sprouting cumulus cloud with a smooth, caplike cloud above it.

Examples of common species

What follows are descriptions of some common cloud species, identified by both their genus and species names.

Stratus fractus

Nicknamed "scud," this cloud type is the ragged underside of a stratus or nimbostratus that has separated from the parent cloud above it. Stratus fractus often appears before or after a rain- or snow-shower. The cloud gets its shredded appearance from being blown about by the wind.

Stratocumulus stratiformis

This stratocumulus has puffy cloud segments that have grown together into a solid, thick, lumpy sheet. Stratocumulus stratiformis may produce precipitation heavier than a drizzle, which is unusual for a stratocumulus cloud.

Altocumulus undulatus

Altocumulus undulatus takes the form of parallel rows of altocumulus clouds. They may appear in patches or cover most or all of the sky. Sometimes the rows are very close together and resemble the ripples created by dropping a pebble into still water.

The rows are produced by the action of two stacked vertical layers of air, each moving in a different direction. The upper layer of air is the colder of the two. The warm air then rises to the height of the upper layer and cools, causing moisture to condense from it. At the same time, the cold air descends into the warm layer, causing the water in its path to evaporate, or turn from a liquid into a gas. This evaporation results in cloud-less rows where the cold air has traveled, interspersed with cloudy rows where the warm air has traveled.

A key reference to: Learning to identify clouds

Being presented with a long list of cloud types, with Latin names no less, can prove overwhelming. Here are some helpful hints for identifying clouds.

When you look up at the clouds, first look for general characteristics. Ask yourself these questions: Are the clouds flat or bumpy? Do they form a solid sheet or are they individual with distinct edges? Are they white, light-gray, or dark gray? How much of the sky is covered: Is it completely overcast, can you see patches of blue, or is it mostly blue with a few clouds? Do the clouds appear to be low to the ground or high in the sky?

By comparing your answers with the descriptions given for the ten cloud genera, you should be able to identify the genera of the clouds you're looking at. After becoming reasonably successful at this, you'll be ready to start identifying cloud species. This skill requires a familiarity with the descriptions of common species. When watching the sky, one should carry a good field guide or cloud atlas. These books provide pictures of clouds alongside their names and written descriptions.

Altostratus translucidus

Altostratus translucidus is a moderately thick, featureless cloud cover that produces a visual effect known as "watery sky," sometimes called "watery sun." Watery sun is when the Sun looks like a bright, blurry ball set against a gray backdrop. The Sun appears much as it would when viewing it through frosted glass. A layer of altostratus translucidus is thicker in some parts than in others. The thickest parts of this cloud may totally obscure the Sun.

Cirrus uncinus

This picturesque form of high, ice-crystal cloud looks as if it were painted on the sky with fine brush strokes. Cirrus uncinus, with its series of hook-shaped filaments, has been nicknamed "mares' tails." It also has been described as commas in the sky.

The distinctive look of these clouds is created by the wind. The clouds develop when ice crystals within the clouds grow. The clouds become heavier and begin to descend. They are then whipped by strong winds below, sometimes jet stream winds, and spread horizontally across the sky. The clouds' hooked tails indicate the direction of winds aloft.

Cirrus floccus

Cirrus floccus resemble small cumulus clouds, high in the sky. These delicate, woolly-looking tufts are formed when warm air continues rising past the base of a cirrus cloud, depositing condensation above. Falling ice crystals, which look like hazy veils, trail beneath the clouds and are blown horizontally by the wind.

Cumulus humilis

These clouds are the smallest species of cumulus cloud. They look like small tufts of cotton. Cumulus humilis are formed by weak convection currents. This pattern stands in contrast to other species of cumulus, in which convection is stronger and produces taller clouds. Cumulus humilis is also formed during the breakup of a layer of stratocumulus.

These clouds are relatively flat on the bottom, rounded on top, and wider than they are tall. They are generally associated with fair weather.

Cumulus humilis clouds may grow throughout the day, as the Sun heats the surface and warm air pockets continue rising. They may evolve into cumulus mediocris and cumulus congestus and, under the right conditions, into a cumulonimbus thunderhead.

Fractocumulus

Fractocumulus is a cumulus cloud with tattered edges. Sometimes small fragments begin to break off this cloud. The fragments may hang on the edge of the parent cloud or may separate entirely and hover nearby.

Fractocumulus often represents an intermediate step in the development of a cumulus cloud, either as it is beginning to form or beginning to dissipate. Fractocumulus may appear white or gray, depending on the Sun's position in the sky and the thickness of the cloud.

Unusual clouds

In the following descriptions of some unusual cloud formations, three of the following six examples (pileus, mammatus, and lenticular) have been introduced in the species list above. Each of these formations can occur in multiple genera. The other three examples (banner, Kelvin-Helmholtz, and contrails) will showcase clouds that are formed under special conditions and are not included in our species list.

Pileus clouds

Pileus clouds, also known as cap clouds, are smooth formations found at the top of cumulus congestus or cumulonimbus clouds. A pileus cloud is formed by strong updrafts associated with a growing cumuliform cloud. These updrafts, which reach speeds of 20 to 30 mph (32 to 48 kph), force a parcel of air sharply upward. The air parcel travels along the side of the cumuliform cloud and over the top. It settles above the cloud, where the moisture within the air condenses into a flat, elongated cloud.

If the cumuliform cloud continues to grow taller, it will touch the pileus cloud, making it appear that the cumuliform cloud is wearing a pileus cap.

Mammatus clouds

These distinctive, beautiful formations are often associated with severe weather. They most commonly develop on the underside of the anvil of a mature cumulonimbus cloud. They also form infrequently underneath altostratus, altocumuluscirrus, cirrocumulus, and stratocumulus clouds.

Mammatus clouds are round pouches of moisture. They appear in clusters, hanging down from and covering the underside of a cloud. They typically develop on a cumulonimbus cloud after the worst part of a thunderstorm has passed. Contrary to popular belief, the presence of mammatus rarely signals a tornado. The formation of mammatus beneath cloud types

other than cumulonimbus is a sign that thicker clouds and rain showers are either approaching or retreating.

Mammatus clouds are formed under the unusual conditions of warm, moist air traveling downward, a sort of reverse convection. The process works like this: When a thunderstorm cloud reaches the top (or occasionally surpasses the top) of the troposphere, it quits growing vertically and spreads horizontally, creating an anvil, or flattened formation at the top of a mature cumulonimbus. Pockets of warm air continue to rise to the top of the cloud and travel horizontally along the top of the anvil. Due to the large concentration of ice crystals and water droplets suspended in these pockets of air, they are heavier than the surrounding air and begin to fall.

Normally, air becomes warmer as it descends, causing the moisture within it to evaporate. However, in this case, the falling air pockets contain so much moisture that any heat gained while descending is expended in the process of evaporation. If more heat is lost than gained during the descent, the air pockets actually become cooler than the surrounding air. In that case, the moisture within them condenses once again. When this condensation occurs at the base of the anvil, it forms the structures known as mammatus clouds.

Lenticular clouds

Lenticular clouds are part of a class of mountain-wave clouds, also called orographic clouds. They are generated when moist wind crosses over a mountain range. The wind blows along the surface of the westward, or windward, mountain side of the mountain, over the top, and down the eastward, or leeward side. As a result, a wavelike pattern of winds is set into motion. That pattern continues for several miles downwind of the mountaintop.

As the air moves upward toward the crest of a wave, it cools. If it is carrying sufficient moisture, clouds will form. The air then flows downward toward the trough of the wave, during which it warms and clouds

evaporate. This motion results in the formation of lenticular clouds at the wave's peaks, while clouds are absent in the troughs.

If alternating moist layers and dry layers of air exist above the mountains, these lens-shaped clouds will form one on top of another, resembling a stack of pancakes. As long as the wind continues moving through the wave at a constant rate, the clouds will remain stationary.

Lenticular clouds are a common sight over most mountain ranges. The most spectacular lenticular clouds are formed over the largest mountain ranges, such as the Himalayas, the Andes, and the Rockies.

Banner clouds

Banner clouds also belong to the class of mountain-wave clouds. They are so-named because they look like banners waving from mountaintops.

A banner cloud is also sometimes referred to as a smoking mountain. It forms at a mountain's peak and drapes down over its leeward slope. A banner cloud, like a lenticular cloud, is a product of the wavelike motion of the wind across a mountain range. The reason that a banner cloud forms specifically on a mountaintop, however, has to do with changes in air pressure, or the force exerted by the weight of the air.

A banner cloud forms because air pressure builds on the windward side and subsides on the leeward side of a mountain. The windward high-pressure area is formed by the amassing of air as it travels upward, along a

mountainside. As a result, pressure drops on the windward side. Condensation (and cloud formation) occurs more readily in areas of low pressure.

The most famous banner clouds are those that often adorn the peaks of Mount Everest in the Himalayas and the Matterhorn in the Swiss Alps.

Kelvin-Helmholtz clouds

Kelvin-Helmholtz clouds are cirrus clouds that look like breaking ocean waves or narrow, horizontal corkscrew spirals. They are comprised of a series of eddies (small parcels of air that flow in a pattern different from the general airflow). Since they dissipate within a couple of minutes of forming, they are rarely seen.

Kelvin-Helmholtz clouds are the product of a strong wind shear. Wind shear refers to the rate of change of wind speed, or wind direction, over a set distance. The formation of Kelvin-Helmholtz clouds requires the presence of two vertical air layers of different densities that travel at different speeds. The upper layer must be the warmer and less dense of the two. Given a great enough wind shear, eddies will develop where the two air layers meet.

Kelvin-Helmholtz clouds were named for Scottish physicist Baron Kelvin (1824–1907) and German physicist Hermann von Helmholtz (1821–94), the two scientists who were the first to describe this pattern of eddies in fluids in the late nineteenth century.

Contrails

Contrails are the only cloud type covered in this chapter that are not naturally occurring. These clouds are cirruslike markings spread

across the sky by aircraft flying at 16,500 feet (5 kilometer) or higher. "Contrails" is an abbreviation for "condensation trails."

A contrail is the frozen trail of water droplets emitted by an aircraft's exhaust. At high levels of the troposphere, or low levels of the stratosphere where large aircraft travel, the air temperature is typically below ?68°F (?56°C). At this temperature, water droplets freeze within seconds, appearing white against the blue sky.

When the surrounding air is relatively dry, these artificial clouds are thin, dissipate quickly, and are generally not visible from the ground.

Contrails formed in air with a high relative humidity, however, are thicker than their dry-air counterparts. They may remain visible in the sky for a half hour or more, particularly if winds are light. During this time, they may spread apart, forming a sheetlike cloud.

A thick, persistent contrail is a sign of moisture in the upper air. This sign is often the first clue that a frontal system, a weather pattern that accompanies an advancing front, is approaching.

Cloud identification and forecasting

Certain types of clouds, and especially sequences of clouds that develop over time, are reliable indicators of weather to come. Their usefulness for forecasting is greatly improved when wind speed is also considered. Once one has a familiarity with the basic cloud types, all it takes is regular

observations of the sky to predict when the weather will be fair, wet, or stormy, as well as whether the air will become warmer or colder.

Fair- and foul-weather clouds

The discussions of high, middle, and low clouds earlier in this chapter described which types of precipitation, if any, are associated with each cloud genus. What follows is a summary of the weather implications of the major cloud types.

The fairest-weather clouds are high clouds. These are cirriform clouds composed of ice crystals. They rarely produce precipitation that reaches the ground. When rain or snow does fall from these clouds, it usually takes the form of virga, or fall streaks, rain that evaporates as it falls.

When cirrus clouds thicken, however, they often represent the leading edge of a frontal system, usually a warm front. If this is the case, one can expect to see the formation of progressively lower layers of clouds and precipitation within the next twenty-four hours.

Next comes the middle clouds: altostratus and altocumulus. These clouds often form in advance of a storm. Only when they exist in thick layers will these clouds yield precipitation. The presence of altocumulus castellanus, in the morning on a warm, humid day signifies the likelihood of afternoon thunderstorms.

Low clouds are the most likely to produce steady rain or snow. Stratus may do nothing more than hang in the air like a gloomy, gray blanket, or it may yield light precipitation that sometimes lasts for days. A thick layer of stratocumulus, which is often the leading edge of a cold front, may bring light drizzle or snow. The rainiest of the low clouds is nimbostratus, which often produces continuous, light to heavy precipitation for longer than twelve hours.

The final category, vertical clouds, can do just about anything. They range from cumulus humilis, the fairest of all fair-weather clouds, to cumulonimbus incus, the king of the thunderstorm clouds. The degree of raininess or snowiness of these clouds depends on their degree of vertical development: the taller the cloud, the more likely that it will produce precipitation. When cumuliform clouds yield precipitation, it is usually localized, heavy, and short-lived.

Wind direction

After identifying the types of clouds in the sky, the next step in predicting the weather is to determine the direction of the surface winds. It is also important to note if, and how, the winds are shifting. Wind direction is important because the wind carries along the moisture that produces clouds and precipitation. For this reason, a change in the wind direction usually signals a change in the weather.

Cloud sequences and fronts

The first sign of an approaching cold front or warm front is that cirriform clouds appear high in the sky. Next come middle clouds, then low clouds. These clouds become progressively darker, eventually bringing precipitation. Once the front passes, the sky clears. The specifics of this general sequence of events are different for cold fronts than they are for warm fronts.

Cold fronts

A cold front, the leading edge of a relatively cold air mass, advances more quickly than does a warm front. It generally takes five to seven days to pass completely through an area. The first sign of a cold front is the appearance of cirrus clouds, which remain in the sky for as long as several days.

After that, one can expect the development of any number of sequences of clouds. This pattern depends on the speed with which the front is moving. In general, however, the next type of cloud to form is cirrocumulus, followed by either altocumulus or altostratus. Then comes a layer of stratocumulus which becomes progressively thicker. Rain clouds, nimbostratus or cumulonimbus, are next on the scene.

The intensity of precipitation brought by the rain clouds depends on the strength of the front. The strength of the front, in turn, is determined by the contrast in temperature between the two air masses: the colder air mass being ushered in by the cold front and the warmer air mass that is being displaced. Storms are fueled by the contrast of warm and cold air. The greater the temperature difference, the more severe the storm.

Thus, the final stage of a cold front's passage through an area can be marked by anything from an overcast sky and drizzle to a series of two or three thunderstorms. When the front has passed, blue skies and colder air remain.

Warm fronts

A warm front moves along more slowly than a cold front and is much gentler in its approach. Whereas a cold front pushes its way into a warm air mass, thrusting the warm air above it, a warm front overtakes a cold air mass by gliding above the cold air and nudging it along. The upper, leading edge of a warm front can arrive at a given location up to 1,000 miles (1,600 kilometers) and several days before its base arrives.

Because a warm front moves so slowly, its leading edge produces an extensive sequence of cloud types. Initially there are wispy cirrus clouds. These spread out across the sky into cirrus spissatus. Next comes a layer of either rippled cirrocumulus (called "mackerel sky") or cirrostratus. After that come the middle clouds: altostratus followed by altocumulus. As moisture continues to condense at lower levels, stratocumulus and sometimes stratus appear.

As the base of the warm front approaches, rain clouds—nimbostratus or, infrequently, cumulonimbus—form. Whereas the precipitation associated with a warm front is usually lighter than that associated with a cold front, it can last for several days, and it may be heavy at times. When the front has passed completely, the sky clears and the temperature rises.

Weather report: Keep a cloud journal

Try your hand at forecasting by the clouds. Each morning and evening for a week, make a sketch of the clouds you see and attempt to identify them by genus and species. For each journal entry, make predictions about which types of clouds will appear next, as well as whether it will rain or snow. Your predictions should be based on the sequence of cloud types that have appeared since the start of your journal. For each entry, note whether your previous prediction came true.

[See AlsoWeather: An Introduction ]

For More Information

BOOKS

Day, John A. The Book of Clouds. New York: Silver Lining Press, 2003.

Day, John A. Peterson First Guide to Clouds and Weather. Boston: Houghton Mifflin, 1991.

Hamblyn, Richard. The Invention of Clouds: How an Amateur Meteorologist Forged the Language of the Skies. London: Pan MacMillan, 2001.

Pretor-Pinney, Gavin. The Cloudspotter's Guide. New York: Perigree Trade, 2006.

World Meteorological Organization. International Cloud Atlas. Geneva, Switzerland: World Meteorological Organization, 1987.

WEB SITES

University of Illinois. Cloud Catalog. ?http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cld/cldtyp/home.rxml? (accessed August 15, 2006).

USA Today. Clouds and Fog. ?http://www.usatoday.com/weather/wcloud0.htm? (accessed August 15, 2006).

Utah Education Center. Cloud Guide. ?http://www.uen.org/utahlink/weather/clouds/cloud_id.html? (accessed August 15, 2006).