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).

Clouds

by Aristophanes

THE LITERARY WORK

A Creek play set in Athens in the early fifth century bce; first performed In 423 bce but surviving only in the revised version of 418-416 bce

SYNOPSIS

The somewhat bawdy political satire demonstrates the dangers that Socratic philosophy poses to Athens,

Events in History at the Time the Play Takes Place

The Play in Focus

For More Information

Not much is known about the personal life of the poet Aristophanes, whose comic genius is undisputed. The dates of his birth (c. 446 bce) and death (c. 386 bce) are estimates. We do know that Aristophanes was educated at Athens, of which he was a loyal, if consistently critical, citizen. Of the 44 plays ascribed to him, 11 survive. These 11 testify to Aristophanes’ talent for biting public satire. In addition to Clouds, which lambastes the philosopher Socrates and his followers, Aristophanes’ other works are all ruthlessly critical of various aspects of Athenian society and public policy: The so-called “peace-plays” (Acharnians, Peace, and Ly-sistrata) attack Athens’ seemingly endless war with Sparta. Aristophanes criticizes the corrupt and overused legal system of Athens in Wasps; he condemns the “new” poetry of Euripides in Frogs. And lest we think that Socrates was the only well-known public person to figure so prominently in Aristophanes’ works, another surviving play (Knights) criticizes and even ridicules the Athenian ruler Cleon. In fact, so fierce was Aristophanes’ criticism that Cleon brought a lawsuit against the poet, charging him with slander and treason.

The comedy of Aristophanes’ day typically transgressed acceptable behavior. Still, the coarse, vulgar, and even obscene nature of Aristophanes’ comedies has led to a great deal of speculation about his traits. A few have assumed that the poet-playwright himself must have been fairly crude and somewhat of a buffoon, while others have pointed out the need to distinguish between his morals and manners and those of his characters. With so little information to go on, scholars have been unable to settle the debate about what kind of man Aristophanes was. Most, however, describe him as a man who was conservative, romantic, and even reactionary. Plato’s dialogue the Symposium depicts Aristophanes as circulating in the highest levels of society, a portrayal that many consider fairly accurate. Though his comedies are bawdy and ribald, like the others of his age, the poet had high expectations for his comedies and what they could accomplish. In all his plays, his harsh criticism of the modern approach to values, morals, art, public policy, and ways of viewing the world demonstrates an unequivocal longing for Athens’ “good old days” and a desire to uphold the religious and social conventions of tradition. Aristophanes is said to be the first literary critic, and he judged poetry based on what he thought was its primary function—to make us better human beings. His Clouds aims to save Athens from the “danger” of developments in Greek philosophy.

Events in History at the Time the Play Takes Place

Determining who the real Socrates is

There is no doubt that the character of Socrates in Aristophanes’ Clouds was inspired by the historical Socrates (469-399 bce), the famous philosopher who was tried, found guilty. and sentenced to death by the city of Athens in 399 bce, 24 years after the performance of Aristophanes’ play. Since Socrates did not produce any written documents, we must glean much of what we know of him from the writings of others, especially his student Plato. Plato believed that Clouds was in no small measure responsible for Socrates’ conviction. Although Plato’s Apology reads like a transcript of the trial and seems to have been written fairly soon after Socrates’ death, scholars disagree about how much of the dialogue records actual events and how much artistic license was taken. Another friend of Socrates, Xenophon, wrote his own Apology, and its account of the trial differs. In any case, Plato’s Apology condemns the way that Aristophanes portrayed Socrates as irresponsible and characterizes him very differently than Clouds.

In Plato’s Apology, Socrates begins his defense to the jury by outlining the charges against him. He divides his accusers into two groups, the “old” and the “new.” The new accusers are the men who have charged him with the specific, “official” offenses for which he is on trial: not believing in the city’s gods and corrupting the youth of Athens. But the old accusers, according to Socrates, are far more dangerous; they have been slandering him and turning public opinion against him for years. Socrates refers to a “certain comic poet,” to whom members of the jury listened when they “were most trusting, when some of you were children and youths” (Plato, Apology, 18d). Socrates laments that this comic poet (Aristophanes) had more than 20 years to malign him, while Socrates had only one day to change their minds. He summarizes the charges leveled against him:

“Socrates does injustice [commits a crime], and is meddlesome, by investigating things under the earth and the heavenly things, and by making the weaker speech the stronger, and by teaching others these same things.” It is something like this. For you yourselves also used to see these things in the comedy of Aristophanes: a certain Socrates was carried around there, claiming that he was treading on air and spouting much other drivel.

(Apology, 19c)

In response, Socrates simply states “None of these things is so” (Apology, 19e). He asks if any jury member has ever heard him conversing about these topics, and if so, to come forward. None can, he adds, because he never discussed the things Aristophanes has accused him of, and the same holds true for the rest of Aristophanes’ rumors about him.

Based on what we know of Socrates from Plato and other sources, Aristophanes’ portrayal is not entirely accurate. First, Plato’s Socrates is not concerned with the physical sciences and investigations into material causes, although he admits in another dialogue by Plato (Phaedo) that he pursued such inquiries as a youth. Second, Plato’s Socrates never taught or purported to teach anyone how to “make the weaker speech the stronger.” Moreover, he, in contrast to the sophists, did not place high value on wealth. He did refute prominent men in public discussions, often making them look ridiculous in the process. He admits too that some of his followers learned to speak cleverly by his example. And he is indeed revolutionary in pushing for a new understanding of the virtues, which is no longer founded on the traditional myths about the Greek gods. However, Plato’s Socrates also champions justice and virtue, while Aristophanes’ Socrates in Clouds is a pompous charlatan who undermines traditional morals and values, such as piety and a son’s obedience to his father.

Would Aristophanes have been aware of these differences between his portrayal of Socrates and the actual man? Perhaps he was, but took artistic license with his subject. Aristophanes might have concluded that Socrates would make a useful scapegoat and prime example of all that was wrong with the sophists and philosophers. According to the Apology, Aristophanes’ play did great damage to Socrates’ case. There are scholars, however, who counter that “the power of Aristophanic satire to influence political events is grossly overestimated” and “no firm evidence can connect the philosopher’s condemnation and the comic poet’s caricature” (Segal, p. 45). According to this view, Socrates merely provided great material for comic inspiration. Not only was he well known in Athens as an eccentric who went around barefoot questioning people, but, as already noted, his unusual physical characteristics (such as his odd gait) made him ideal for comic caricature.

The conservative poet and a city in flux

When Aristophanes wrote his plays, Athens was undergoing a dramatic transformation. Advances were being made in art, architecture, music, literature, science, and philosophy, and these advances posed serious challenges to established religious tradition and its associated morality and ethics. Militarily, Athens took an ever more aggressive stance, pursuing a policy of expansion and internationalism that brought a flood of merchants, traders, and soldiers to the city, all foreigners who introduced the Athenians to new beliefs, value systems, and worldviews. Tributes from a growing empire and the discovery of silver deposits outside Athens, at Laurion, further enriched the city. In keeping with these developments, the pursuit and accumulation of wealth became a central concern for an increasing number of people. These are all changes that Aristophanes perceived as threats to the old, established order in Athenian society and that he criticized in his works.

In such an atmosphere, it is easy to see why Aristophanes would have viewed Socrates and his philosophy as a serious threat to Athenian society. Socrates did not believe in accepting conventional wisdom or public opinion on any given subject. His whole method of philosophy was one based on questioning. As practiced by Socrates, philosophy consisted of verbal exchanges, wherein an important query such as “What is the nature of justice?” was discussed with conversation partners or interlocutors in a series of question-and-answer sessions. In the Apology, Socrates claims that of all the Athenians he has questioned, the politicians and the poets have been the most ignorant. Not only do they know nothing about justice or politics or the gods; they pretend that they do! Socrates reasoned that he was more knowledgeable than the Athenians he questioned: “I am wiser than this human being. For probably neither of us knows anything noble and good, but he supposes he knows something when he does not know, while I, just as I do not know, do not even suppose that I do” (Apology, 21d). For Socrates, wisdom amounted to recognizing one’s ignorance. This realization marked the beginning of the quest for the truth, which required one to question and even to blatantly disregard established opinions and myths. No doubt the spirit of rebellion in this approach alarmed Aristophanes, a poet who has been characterized as “The great reactionary who opposes with all the means at his disposal all the new-fangled things, be it the democracy, the Euripidean tragedy, or the pursuits of Socrates” (Strauss, Rebirth, p. 103).

The sophists

Traveling scholars and intellectuals known as sophists (derived from the Greek term for “wisdom”) traveled the Greek world, lecturing and teaching for a fee mathematics, history, natural philosophy, rhetoric, and more. Too diverse in their interests and approaches to be considered a unified movement, sophists elicited both positive and negative reactions from the cities they frequented. Several sophists were highly regarded, and many had successful careers. The Athenian statesman Pericles took pride in his friendships with the sophists Damon and Anaxagoras, both active during the fifth century bce; also he invited another well-known sophist, Protagoras of Abdera (c. 481-420 bce), to Athens.

Other Athenians considered sophists a disruptive and possibly dangerous element in society, mainly because of their argumentative skills, which could be used to undermine or at least call into question the validity of traditional religion, morality, and government. Many also feared the sophists’ possible influence on the aristocratic youth of fifth-century Athens. Three of Socrates’ former disciples—Alcibiades, Critias, and Charmides—became involved in anti-democratic activities, colluding with Sparta, Athens’ enemy in the Peloponnesian War, and initiating a coup known as the reign of the Thirty Tyrants. When democracy was restored in Athens, Socrates him-self fell under suspicion.

Those hostile to the sophists, like Plato, tended to depict them mainly as clever rhetoricians, convincing speakers who used the art of persuasion to win any argument, without any regard for right and wrong. Indeed, the modern term “sophistry” is often applied to clever, specious arguments.

Probably the public at large found it hard to distinguish between the kind of arguments the sophists were famous for and what Socrates referred to as his questioning of Athenian citizens in pursuit of the truth. At the time, there was no clear-cut distinction between the two types of discussions, so Socrates tended to be tarred with the same brush as the sophists. If Aristophanes shared the prejudices of his fellow Athenians, he might have truly believed Socrates to be a sophist and thus have focused on all that the philosopher had in common with that disreputable group: intellectual pretension, an outlook that embraced moral relativism rather than strict morality, and a skepticism that made people question basic values, like belief in the gods and obedience to the laws.

The Play in Focus

Plot summary

Aristophanes’ Clouds uses its main character to present Socrates and his philosophy as a corrupting and dangerous influence on Athens. Strepsiades, the main character, is a simple, traditional farmer from the countryside who has married unwisely, relates poorly with his son, and is struggling with huge debts because of his family’s extravagance. His wife, a “classy, luxurious, aristocratic” townswoman, has very expensive tastes (Aristophanes, Clouds, line 48). He blames her not only for spending so freely but also for spoiling their son, Phidippides, whose costly habits she encourages. The son dabbles in horses and gambling, indulging his taste for racing with an abandon that has thrown his father into debt. After a night of fretting, Strepsiades comes up with a plan: he decides that he and his son will attend Socrates’ school, which is known as the thinkery, to learn how to make the “weaker speech the stronger” and thus convince his creditors to forgive his debts.

At the thinkery, a student of Socrates recounts the brilliant investigations conducted by his teacher that very day. Socrates figured out how many of its own feet, or how far, a flea could leap when it jumped from person to person by making wax slippers for the flea and measuring them. Also he sought to discover whether “gnats hum through their mouth or through their be-hind” by examining the intestines of a gnat, after which he declared that the “gnats’ anus is a trumpet!” (Clouds, lines 158-159). When Strepsiades wonders why a group of young men are bent over with their own anuses looking at heaven, the student replies that their anuses are learning astronomy. As if these descriptions of Socrates and his pursuits were not derogatory enough, the student makes it known that Socrates, while “gaping upwards” as he investigated “the courses and revolutions of the moon” was “crapped on” by a lizard from the roof (Clouds, lines 193, 171-173).

Not only does Aristophanes portray Socratic investigations as useless and ludicrous in the extreme, but also as distracting Socrates and his students to such an extent that they forget even their most basic needs. The student tells Strepsiades that Socrates was so engrossed in his studies the previous evening that he forgot to prepare or get them dinner, so they all went hungry. In much the same way, philosophy seems to have distracted Socrates and his students from Athenian politics; the student has no idea what Strepsiades is talking about when he refers to the Peloponnesian War. Philosophy, as Aristophanes depicts it, is concerned only with scientific knowledge and universal definitions and concepts. It posits a Socrates who seeks a philosophic detachment from the affairs and concerns of the city. In fact, when the play first introduces Socrates, he is physically detached from the city. He comes into view suspended in a basket in the air, attempting to emulate the free and formless clouds, which he chooses to worship instead of the traditional gods. For Aristophanes’ Socrates, “Zeus does not even exist … the true gods are the Clouds” (West in Aristophanes, p. 29).

Aristophanes makes his attack on philosophy even more explicit in the subsequent scene. When Phidippides, the son, decides to enter the thinkery, the characters representing Just Speech (an old man) and Unjust Speech (a young man) have a contest to determine which will have the privilege of educating him. Just Speech blames Unjust Speech for the current corruption of the city’s youth, who no longer have respect for their parents, the gods, or the city and its laws. Moderation and restraint are the behaviors to practice, says Just Speech. In the good old days, when Just Speech was the norm, the youth were silent, respected their elders, and became courageous and strong through gymnastics and athletics. Un-just Speech counters with arguments of its own. It warns the young man of all the pleasures he will have to give up in the name of moderation: lovers, drinking, gambling, delicacies. If he acquires Unjust Speech, he will be able to act however he wants. Next, Unjust Speech points out contradictions in the various elements of religious traditions. It ends its “pitch” with a flourish, pointing to the audience and stating that even the leading men of Athens, its orators, lawyers, and poets, no longer live by the beliefs that Just Speech espouses. Not knowing how to respond, Just Speech concedes defeat.

The next time we see Phidippides, he has re-turned home from the thinkery after completing his study of the unjust speech. He instructs his father in what arguments to make to his creditors in order to avoid paying his debts. Strepsiades manages to use these rhetorical tricks successfully, which makes him deliriously happy until he gets into an argument with his son. The next scene finds him running from the house and hurling names at his son, calling him a wretch and a father beater. Apparently Phidippides physically assaulted his father during an argument, and when Strepsiades told him to stop, the son re-fused, claiming he could use his argumentative powers to persuade his father that parent-beating is just.

At this point, Strepsiades realizes the consequences of what he has done and the full ramifications of life in a society that throws off its laws and traditions. Distraught, he asks the god Hermes what to do and is told to burn down Socrates’ thinkery. Fire is the only way to destroy it; lawsuits or other civil remedies will not do

THE PELOPONNESIAN WAR

The war between Athens and Sparta from 431 to 404 bce was the most important and possibly the most troubling aspect of Athenian policy during Aristophanes’ lifetime. During the war, Sparta besieged Athens, forcing its citizens to remain within the city’s defensive walls, which the Spartans could not breach. The plague that resulted from those crowded, unhealthy conditions claimed about a quarter of the Athenian population, including the leading statesman Pericles. Aristophanes criticized the war and its political supporters in no fewer than three plays, the most famous of which is Lysistrata (41 1 bce), a hilarious comedy wherein the women of Athens con-spire to end the war by withholding sex from their husbands (also in Classical Literature and Its Times). In Clouds, the main character, Strepsiades, often refers to this war, most humorously when he asks one of Socrates’ students to move Sparta farther away from Athens on the map they are studying.

because the philosophers can simply talk their way out of them. Heeding the advice, Strepsiades sets the edifice ablaze to rid society of the disease of unjust speech.

Aristophanes’ true targets

While Aristophanes’ play makes Socrates the butt of the satire, it in fact attacks pretentious intellectuals in general for their impact on daily life in Athens. Using Socrates as a representative appears to have been a practical choice, since most of the other intellectuals were foreign migrants to Athens and so less familiar to the Athenian public. Also Socrates was someone with physical characteristics—a snub nose, large eyes, and an odd gait—on which a comedy could capitalize. Moreover, he was known for his habit of drawing people into conversations in public. Besides employing these character traits, Aristophanes assigned to Socrates “all the intellectual theories and activities which he wished to ridicule”; that some of them did not really apply to Socrates mattered little to him (MacDowell, p. 132).

Aristophanes’ reasons for making Socrates his comic scapegoat apparently stem from a harmful legal development in the city, for which the playwright blamed the intellectuals. Many Athenians were bringing weak cases before the courts and setting out to win nonetheless. By Aristophanes’ day, it had become possible for the side with the weaker evidence to prevail if the evidence was presented cleverly enough. Plato’s student Aristotle described such a scene, using the example of a man accused of assault: a physically weak man charged with assault might argue that his weakness made him an unlikely culprit; charged with the same crime, a strong man could argue that he would not have committed it be-cause others would immediately suspect him on account of his strength. Aristotle notes that both arguments appear probable, but the strong man’s argument only seems to make sense because of the way it is presented: “[T]his is what making the worse argument the better is” (Aristotle in MacDowell, p. 128).

Such attempts at legal wrangling were attributed, with some justification, to the teachings of the sophists. Protagoras, perhaps the first man

OLD COMEDY, MIDDLE COMEDY, NEW COMEDY

Toward the end of Aristophanes’ career, Old Comedy fell into a serious decline. Aristophanes’ last plays, particularly plutus (388 bce), heralded the beginning of “Middle Comedy,” in which the chorus was less involved with the actors (as was already in the case in tragedy) and the satire targeted political figures less often. In the fourth century bce, the poet Menander completed the transformation of the genre into “New Comedy,” wherein luck and fortune provide plenty of surprises and plottwists. Cone were the biting satire and scathing criticism of Aristophanes; Menander replaced them with more subtle observations about people and human nature in what can best be described as comedies of manners. Significantly, Menander’s career covered a period in which democratic rule was replaced by an oligarchy in Athens. The more biting or critically laced comedy would reappear once the democracy was restored.

to identify himself as a professional sophist, be-came most widely associated with the claim that he could teach people “how to make the worse argument the better” (MacDowell, pp. 128-129). He was also credited with the authorship of a treatise on argumentative techniques. However, Protagoras was not universally condemned as a malign influence. He also taught the art of citizenship, arguing that, whatever sorts of things seem just and honorable to a given city are in-deed that way as long as the city deems them so. Others of the age condemned Protagoras for not drawing an absolute distinction between truth and falsehood, or right and wrong. While Protagoras employed persuasion to a higher purpose, hoping to develop good citizens, his followers—who arrived in Athens during the 420s—perfected the art of persuasion for its own sake. Several used it to excuse terrible crimes, arguing, for example, that if a king wins power (the end), it justifies his having killed or banished others (the means) to attain it.

Sources and literary context

Although Clouds does not name real individuals, it “contains more or less clear allusions to theories and activities which we can attribute to Anaxagoras, Protagoras, Diogenes, and Antiphon,” among others (MacDowell, p. 130). The poetic comedy furthermore reflects a very real competition between poetry and the fledgling field of “philosophy” (not yet known by that name when Clouds was written). The poets of fifth-century Athens vied with the newer intellectuals for the position of supreme dispensers of wisdom in society. Aristophanes manipulates it so that poetry, in the form of his play, emerges the victor.

Aristophanes’ plays are among the earliest surviving comedies in Western civilization; we only have descriptions or fragments of earlier come-dies. The particular comedic style of fifth-century Athens is referred to as “Attic Comedy” or some-times as “Old Comedy.” The Greek komoidia, from which “comedy” derives, refers to a song, often sung by a group of costumed men who would entertain audiences at festivals or other public events. Not much is known regarding the trans-formation of komoidia into comic plays, but apparently Old Comedy preserved the antagonistic relationships between characters (or between a character and the chorus), as well as the biting social and political critiques of the costumed men. The plays of Old Comedy seem to have revolved around issues of the day. In the fifth century bce, comic plays were directly political in a way that tragedy was not; their jokes had a bite and were often meant to be taken very seriously. Although the comedies generally featured a fantastic plot, they were “part of a real democracy” (Boardman, et al, p. 168). They commonly ridiculed key figures in society or portrayed them as villains, and also showed irreverence for myths and other sacred beliefs. The chorus, made up of 24 members, figured prominently in Old Comedy, playing a dramatic role and at times speaking directly to the audience for the poet.

Along with its cutting edge, Old Comedy had some standard lyrical and humorous elements. It contained beautiful melodies and meters as well as base verses that referred to bodily functions, such as vomiting or evacuating one’s bowels. Costuming and staging contributed to the comic effect. It is not only “the chamber pot [toilet] that is so omnipresent,” in the comedies but the phallus as well, notes one scholar (Segal, p. 36). In Old Comedy all the actors, even those portraying gods, wore costumes featuring a dangling phallus, which could be manipulated by a string to indicate sexual excitement.

Performance and impact

Aristophanes’ come-dies and those of his peers were performed at festivals for Dionysus, god of wine and merrymaking. These festivals, each of which lasted several days, were state holidays. They were held twice a year, in spring and winter, and they included religious rites such as prayer and an animal sacrifice. In Aristophanes’ age, Athens went to great effort to finance lavish performances of plays at the festivals. Men wealthy enough to finance a dramatic production were required by law to do so at least once in their lives. Athens’ theater could hold around 17,000 spectators—perhaps half the citizenry. Among the festival events were various processions and musical competitions, but most important was the staging of tragic and comic plays. Normally three tragedy writers would compete in a festival, each of them producing several plays, and five comic poets would compete, each with a single play.

Unfortunately for Aristophanes, Clouds did not do as well as he hoped, placing third (last) in the Great Dionysia festival, a fact that the Chorus laments in the later, revised version of the play that survives. We have no explanation of why the judges relegated the play to third place, but Socrates had recently behaved bravely on Athens’ behalf in the real-life Battle of Delion, so it may have been the wrong time for a play that ridiculed him, although there were other plays that did (Dover, p. 119). Sorely disappointed, Aristophanes, who regarded Clouds as his finest work, began to revise the play in hopes of being given another chance to present it at a festival. This hope probably never materialized, since plays were seldom performed twice. Most scholars agree that the version that has come down to us is a partially revised second draft.

Along with the rest of Aristophanes’ surviving plays, Clouds is still regularly performed, some-times with modern touches. A production by the Ancient Comic Opera Company in Toronto, for example, set Aristophanes’ lyrics to the music of Gilbert and Sullivan, Wagner, and Mozart. The production also characterized Phidippides in a way that included impersonations of popular screen stars (Johnny Depp, Jim Carrey, Keanu Reeves, and William Shatner), and it referred to the television game show Jeopardy. Such adaptations testify to the continuing popularity of the playwright and his works.

—Despina Korovessis

For More Information

Aristophanes. Aristophanes’ Clouds. In Four Texts on Socrates. Trans. Thomas G. West and Grace Starry West. Ithaca, N.Y.: Cornell University Press, 1984.

Boardman, John, Jasper Griffin, and Oswyn Murray, eds. The Oxford Illustrated History of Greece and the Hellenistic World. Oxford: Oxford University Press, 1986.

Dover, K. J. Aristophanic Comedy. Berkeley: University of California Press, 1972.

Grant, Michael. The Classical Greeks. New York: Charles Scribner’s Sons, 1989.

MacDowell, Douglas M. Aristophanes and Athens. Oxford: Oxford University Press, 1995.

Nichols, Mary P. Socrates and the Political Community: An Ancient Debate. New York: State University of New York Press, 1987.

Plato. Apology. In Four Texts on Socrates. Trans. Thomas G. West and Grace Starry West. Ithaca, N.Y.: Cornell University Press, 1984.

Segal, Erich. The Death of Comedy. Cambridge, Mass.: Harvard University Press, 2001.

Strauss, Leo. The Rebirth of Classical Political Rationalism. Chicago: University of Chicago Press, 1989.

_____.Socrates and Aristophanes. Chicago: University of Chicago Press, 1966.

Clouds

Classification

Cloud categories

Nimbus category

High clouds

Middle level clouds

Low level clouds

Unusual clouds

Resources

Clouds are condensed atmospheric moisture in the form of minute water droplets or ice crystals. The creation of a cloud begins at ground level. The sun heats Earth’s surface and the warm ground heats the air, which rises and carries with it variable amounts of water, as vapor, that has evaporated from bodies of water and plants. Air at ground level is denser than air at altitude, and as the warm air rises, it expands and becomes less dense. As expansion cools the air, the water vapor that is present in the air condenses into tiny microscopic droplets. Cloud formation depends on how much water is in the atmosphere, the temperature, the air current, and topography. If there is no water, no clouds can form. If condensation occurs below the freezing point, the cloud is made of ice crystals. Warm and cold air fronts, as well as topography, can control how air rises. Clouds that form during vigorous uplift of air have a tall, stacked appearance and clouds formed by gentle uplift of air currents have a flat or stratified appearance. One can make short-term forecasts by observing clouds, as any change in the appearance of a cloud indicates a change in the weather.

Classification

Luke Howard (1773–1864), an English pharmacist and amateur naturalist, developed a system of classification for clouds in 1803. Howard categorized clouds into three major groups using words borrowed from Latin: cumulus (accumulate or piled up heaps and puffs), cirrus (fibrous and curly), and stratus (stretched out and layered). To further describe clouds, he combined those terms and used other descriptive words such as, alto (high), and nimbus (rain). The International

Cloud Classification used today is based on Howard’s system.

There are three basic forms of clouds: cirrus, cumulus, and stratus. All clouds are either purely these forms, or a combination or modification of the basic forms. Because there is more water vapor at lower elevations, lower clouds appear denser than higher clouds.

Cloud categories

Today, there are 10 characteristic forms or genera of clouds recognized by the International Cloud Classification, and there are three height categories with an established altitude range for each category. Low-level clouds range from the surface to 6, 500 ft (2, 000 m), mid-level from 6, 500–23, 000 ft (2, 000–7, 000 m) and high-level, generally above 20, 000 ft (6, 000 m). Below is a brief description of each category and their genera.

Nimbus category

There are two genera of rain clouds, cumulonimbus and nimbostratus. Nimbostratus clouds are usually mid level clouds, thick, dark, gray, and sometimes seen with virga or skirts of rain trailing down. These clouds consist of water droplets that produce either rain or snow. Cumulonimbus clouds are thunderstorm clouds, and arise from cumulus clouds that have reached a great height. When the cloud normally reaches the height of the tropopause, it flattens to resemble an anvil. All phases of water—gas, liquid, and solid—are contained in these clouds, along with powerful updrafts and downdrafts that can create violent storms.

High clouds

The altitude range for high clouds is 16, 500– 45, 000 ft (5, 032–13, 725 m) but they typically form between 20, 000–25, 000 ft (6, 000-7, 500 m). There are three genera of high-level clouds and they are all labeled with the term cirrus. Cirrus clouds are the highest clouds, forming around 30, 000 ft (9, 150 m). They are totally made of ice crystals because they form where freezing temperatures prevail. Pure cirrus clouds look wispy, with a slight curl, and very white. Because of their appearance, they are often called mares’ tails. Cirrocumulus clouds, the least common

cloud, are small, white or pale gray, with a rippled appearance. Sometimes they appear like a sky full of fish scales; this effect is called a mackerel sky. These clouds usually cover a large area. They form around 20,000—25,000 ft (6,000—7,500 m) and are made of either supercooled water droplets or ice crystals. Cirrostratus clouds also form at 20,000—25,000 ft, but are made completely of ice crystals. They usually cover the sky as a thin veil or sheet of white. These clouds are responsible for the halos that occur aroundthe sun or moon.

Middle level clouds

The mid—level clouds at 6,500—23,000 ft (2,000—7,000 m) typically have the prefix ℌaltoℍadded to the two genera in this category. Altostratus clouds appear as a uniform bluish or gray sheet covering all, or large areas of the sky. The sun or moon may be totally covered or shine through very weakly. These clouds are complex as they are usually layered, with ice crystals at the higher, top layers, ice and snow in the middle, and water droplets in the lower layers. Altostratus clouds often yield precipitation. Altocumulus are elliptical, dense, fluffy balls. They are seen as singular units or as closely bunched groups in a clear sky.

Low level clouds

There are three genera in the low level (surface to 6, 500 ft [2, 000 m]). Stratus clouds are usually the lowest of the three genera. Stratus clouds blanket the sky and usually appear gray. They form when a large mass of air rises slowly and the water vapor condenses as the air becomes cooler, or when cool air moves in over an area close to ground level. These clouds often produce mist or drizzle. Fog is a stratus cloud at ground level. Cumulus clouds have flat bases, are thick, and appear puffy. Inside a cumulus cloud are updrafts that create the cloud’s appearance. They form when a column of warm air rises, expands, cools, and condenses. Cumulus clouds occur primarily in warm weather. They consist of water droplets and appear white because the sunlight reflects off the droplets. Thick clouds appear darker at the bottom because the sunlight is partially blocked. Cumulus clouds can develop into cumulonimbus clouds. Stratocumulus clouds are large, grayish masses, spread out in a puffy layer. Sometimes they appear as rolls. These clouds appear darker and heavier than the altocumulus cloud, and can transform into nimbostratus clouds.

Unusual clouds

Beside the basic cloud types, there are subgroups and some unusual cloud formations. Terms such as humulus (fair weather) and congestus (rain shower) are used to further describe the cumulus genus. Fractus (jagged), castellanus (castle shaped), and uncinus (hook shaped) are other descriptive terms used together with some basic cloud types. In mountainous regions, lenticular clouds are a common sight. They form only over mountain peaks and resemble a stack of different layers of cloud matter. Noctilucent clouds form only between sunset and sunrise, and are only seen in high latitude countries. Contrails (condensation trails) are artificial clouds formed from the engine exhaust of high altitude aircraft.

See also Weather forecasting; Weather modification.

Resources

BOOKS

Ahrens, Donald C. Meteorology Today. Pacific Grove,Calif.: Brooks Cole, 2006.

Pretor-Pinney, G. The Cloudspotter’s Guide. New York: Perigee, 2006.

Christine Minderovic

Clouds

Clouds are made of very small drops of water of water, ice crystals, and other small particles in the atmosphere (mass of air surrounding Earth). The water comes from condensation, a process that allows small drops of water to form as the air cools. Cloud shapes and the way clouds form give scientists important clues about local weather and conditions in the atmosphere around the world.

Clouds are divided into several types or families of clouds. These families of clouds are named according to where or how they form, and include high-level clouds, middle-level clouds, and low-level clouds. In addition to belonging to a family, clouds are also named for their shape. Puffy clouds are known as cumuliform clouds, and flat sheet-like clouds are known as stratoform clouds.

How clouds form

In general, as warm, moist air rises upward through the atmosphere, the air cools. As the air cools, ice crystals or water drops appear and clouds form. Meteorologists (scientists who study weather and climate) name clouds based on how they form, where they form, and the shape of clouds. Cloud classifications are organized into groups or families.

Families and types of clouds

The altitudes (heights above the ground) used to describe cloud families change and become lower as one moves from the equator toward the North or South pole. As one moves north or south from Earth's equator (imaginary circle around Earth between the North and South Pole), high altitude family clouds can be observed at much lower altitudes.

High level clouds include cirrus, cirrostratus, and cirrocumulus clouds. These clouds are found at altitudes between 16,000 and 45,000 feet (4,877 and 13,716 meters) above the ground. In comparison, a jumbo passenger jet usually cruises at about 36,000 feet (10,973 meters) above the ground.

Middle level clouds include altostratus, altocumulus, and nimbostratus clouds, and are found between 6,500 and 22,000 feet (1,981 and 6,706 meters) above the ground. These clouds include include altostratus, altocumulus, and nimbostratus clouds. As with many cloud families, the altitudes are not exact, and they can vary depending on the type of terrain (sea or mountains) over which the clouds form or travel.

Low-level clouds include stratus and stratocumulus clouds that are found below 6,000 feet (1,829 meters).

Clouds that form rapidly in a vertical (up and down) direction are known as vertical development clouds. Vertical development clouds include cumulus and cumulonimbus clouds, which are the clouds that form a thunderstorm. Vertical development clouds form rapidly as air rises from the Earth's surface. They are found anywhere from the surface of the ground to 45,000 feet (13,716 meters). In some very strong thunderstorms, the clouds may reach even higher. One factor that contributes to the development of thunderstorms is unstable warm and humid air that quickly rises through the atmosphere to great heights where the surrounding air is very cold.

Shape and color of clouds

The shape of a cloud is determined by the manner in which the water drops condense and the forces of winds that can act to tear away pieces of the cloud as it builds and moves in the atmosphere.

Whether a cloud is light or dark depends upon how much light can pass through the cloud. Water droplets bend or block light. Thicker clouds block more light than thinner clouds, and so appear darker than thinner clouds.

Names of clouds

When clouds are widely separated from other members of their family, the term fracto is added to their name. When a cloud produces rain (precipitates) it is also called a nimbus cloud and the term nimbus is added to the cloud name.

High clouds Cirrus clouds occur at high levels and are usually wispy and long.

If air rises directly upward through the atmosphere, the air cools very quickly. As the air cools, ice crystals or water drops appear and cumulus clouds form. Cumulus clouds are billowy, puffy clouds that resemble cotton balls.

Stratus clouds look as if they are blankets or layers of clouds.

Because it is very cold at high altitudes, high clouds including cirrus clouds, cirrostratus clouds, and cirrocumulus clouds are composed of ice crystals. Particles of dust or pollution often form the center around which the ice crystals grow. For this reason, dust or particles of pollution are often called centers of crystallization if ice grows around them, or condensation nuclei (nuclei meaning the center) if water drops form around them.

Cirrus clouds often produce a shape that looks like a horse tail. These "mares' tails" are wisps of ice crystals. Cirrostratus clouds, because they are thin and because their ice crystals act to both reflect and bend sunlight, sometime appear to form a circle or halo around the Sun or Moon.

Cirrocumulus clouds often appear as patch-like thin clouds.

Middle level clouds Middle level clouds include altostratus clouds, altocumulus clouds, and nimbostratus clouds. These clouds are composed of water drops with some ice crystals near the top of the clouds.

Ice in the Air, Pilots Beware!

Water in the atmosphere can present special dangers to aircraft. For this reason the ability to identify cloud types is an important skill for pilots. In addition, special weather forecasts prepared by aviation meteorologists (scientists who study weather) help pilots avoid dangerous conditions.

When water droplets hit the cold surface of airplanes, ice can form. Ice that forms on wings changes the shape of the wing and can lower or destroy the ability of a wing to produce lift, the force that acts against gravity to allow an airplane to fly.

Ice can also change the shape of key parts of an airplane that allow pilots to control whether the aircraft goes up or down (the elevators), turns left or right (the ailerons), whether the nose moves left or right (the rudder), or the aircraft flies at slower speeds when taking off or landing (the flaps). In every case, ice changes the shape of the surface of these controls and can thus, interfere with a pilot's ability to control aircraft.

Flying through clouds can also be an interesting experience, and not all clouds are dangerous. If the weather is warm enough, and the altitude low enough, the danger of ice forming on the aircraft is low. In addition, there are different types of ice (smooth, rough). Some small amount of frost are normal and not dangerous. Before flight, chemicals are regularly used to remove ice and to help keep ice from forming on airplanes. During flight, special heaters are regularly used to help keep ice from forming on sensitive instruments and parts of airplanes.

Sometimes both middle level and low level clouds contain water that is still in liquid water drops even though the air around them is well below the freezing temperature. This super-cooled water (water below freezing that has not yet formed an ice crystal) needs only a seed, usually a particle of dust or pollution around which to form ice.

At one time, scientists experimented with making rain by seeding clouds with a chemical called silver iodide. It was hoped that the silver iodide would provide a center around which large water droplets would form. When the water droplets grew large enough, they would fall as rain. Cloud seeding thus offered hope that it might be possible to produce rain in dry regions. The results of these early experiments were disappointing, however, and produced little rain beyond the amounts that fell without cloud seeding.

Because of the way ice crystals reflect and deflect light, altostratus clouds often present a bluish-layered appearance. Depending on thickness, altocumulus clouds often have white or gray layers that appear in washboard or wave-like formations. Warm moist air that rises can also result in the formation of castle-like altocumulus castellanus clouds, a form of altocumulus that often appear as isolated cumulous clouds with billowing tops.

Another form of altocumulus cloud is called a standing cloud (properly termed a lenticular altocumulus cloud), and is formed by condensation in currents of air that cool as they move upwards to cross mountains and ridges. Although constantly forming and disappearing, the standing lenticular altocumulus cloud formations appear not to change and thus seem to stand over the mountain or ridge lifting the air.

Nimbostratus clouds often appear as heavy, gray, moisture-laden cloud layers.

Low level clouds Low-level stratus clouds are the gray clouds that often produce rain and some types of fog.

Stratocumulus clouds present the familiar, cotton ball-like cumulus shapes in an elongate form (a cumulus shape drawn out by shearing winds).

Clouds that pass through many levels of the atmosphere, the cumulus and cumulonimbus clouds, often have a widely varying mixture of ice and water. These clouds often have swirling currents of air that move upwards and downwards. These rapid updrafts and downdrafts of air allow ice crystals to appear at much lower levels than normal. As they cycle through the cloud, the ice crystals can grow large enough to fall to the ground as hail.

Although formed from air rising upward from the ground and lower levels of the atmosphere, cumulus clouds often form in fair weather and do not form violent updraft or downdraft currents of air. These cumulus clouds have flat bases and curved tops that look like domes of buildings.

When strong and violent updrafts and downdrafts of air form, however, the air is said to be unstable and the cumulus clouds are said to be more developed. These cumulus clouds have mushroom or cauliflower-like tops, and they often produce rain.

When cycles of air moving upwards and downwards become very violent, cumulonimbus clouds form. Cumulonimbus clouds are dark clouds with anvil-like tops (very flat tops with trailing clouds spreading out like a tabletop) that are often cut off (sheared) by strong winds in the upper atmosphere. Cumulonimbus clouds often have heavy turbulence (rough and violent disturbances of air), rains, lightning, and thunder. The most unstable and violent cumulonimbus clouds can occur in cells or groups capable of forming tornadoes.

K. Lee Lerner

For More Information

Books

Day, John A., et al. Peterson First Guide to Clouds and Weather. Boston: Houghton Mifflin, 1999.

National Audubon Society. National Audubon Society Pocket Guide to Clouds and Storms. New York: Knopf, 1995.

Vasquez, Tim. Weather Forecasting Handbook. 5th ed. Austin, TX: Weather Graphics Technologies, 2002.

Websites

"Meteorology, the Online Guides." Weather World 2010, University of Illinois at Urbana-Champagne Department of Atmospheric Sciences.http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/home.rxml (accessed on August 17, 2004).

"Weather." National Oceanic and Atmospheric Administration.http://www.noaa.gov/wx.html (accessed on August 17, 2004).

Clouds

All clouds are a form of water . Clouds are condensed atmospheric moisture in the form of minute water droplets or ice crystals. The creation of a cloud begins at ground level. The sun heats the earth's surface, the warm ground heats the air, which rises. The air contains variable amounts of water, as vapor, that has evaporated from bodies of water and plants. Air at ground level is denser than air higher up, and as the warm air rises, it expands and becomes less dense. Expansion cools the air and as the air cools, the water vapor that is present in the air, condenses into tiny microscopic droplets. Cloud formation depends on how much water is in the atmosphere, the temperature , the air current, and topography. If there is no water, no clouds can form. If condensation occurs below the freezing point, the cloud is made of ice crystals. Warm and cold air fronts, as well as topography can control how air rises. Clouds that form during vigorous uplift of air have a tall, stacked appearance and clouds formed by gentle uplift of air currents have a flat or stratified appearance. One can make short-term forecasts by observing clouds, as any change in the way a cloud looks indicates a change in the weather .

Classification

A couple of hundred years ago clouds were not identified by name. Luke Howard, an English pharmacist and amateur naturalist, developed a system of classification (from Latin) for clouds in 1803. Howard categorized clouds into three major groups: cumulus (accumulate or piled up heaps and puffs), cirrus (fibrous and curly), and stratus (stretched out and layered). To further describe clouds, he combined those terms and used other descriptive words such as, alto (high), and nimbus (rain). Today, the International Cloud Classification is based on Howard's system.

There are three basic forms of clouds: cirrus, cumulus, and stratus. All clouds are either purely these forms, or a combination or modification of the basic forms. Since there is more water vapor at lower elevations, lower clouds appear denser than higher, thin clouds.

Cloud categories

Today, there are 10 characteristic forms or genera of clouds recognized by the International Cloud Classification, and there are three height categories with an established altitude range for each category. Low-level clouds range from the surface to 6,500 ft (2,000 m), mid-level from 6,500-23,000 ft (2,000-7,000 m), and high-level, generally above 20,000 ft (6,000 m). Below is a brief description of each category and their genera.

Nimbus category

There are two genera of rain clouds, cumulonimbus and nimbostratus. Nimbostratus clouds are usually mid level clouds, thick, dark, gray, and sometimes seen with "virga" or skirts of rain trailing down. These clouds are made of water droplets that produce either rain or snow. Cumulonimbus clouds are thunderstorm clouds, and arise from cumulus clouds that have reached a great height. When the cloud normally reaches the height of the tropopause above, it flattens out, resembling an anvil. All phases of water, gas, liquid, and solid, are contained in these clouds. There are powerful updrafts and downdrafts that can create violent storms.

High clouds

The altitude range for these clouds is 16,500-45,000 ft (5,032-13,725 m) but they usually form between 20,000-25,000 ft (6,000-7,500 m). There are three genera of high level clouds and they are all labeled with the term cirrus. Cirrus clouds are the highest clouds, forming around 30,000 ft (9,150 m). They are totally made of ice crystals (or needles of ice) because they form where freezing temperatures prevail. Pure cirrus clouds look wispy, with a slight curl, and very white. Because of their appearance, they are often called mares' tails. Cirrocumulus clouds, the least common cloud, are small, white or pale gray, with a rippled appearance. Sometimes they appear like a sky full of fish scales; this effect is called a mackerel sky. These clouds usually cover a large area. They form around 20,000-25,000 ft (6,000-7,500 m) and are made of either supercooled water droplets or ice crystals. Cirrostratus also form at 20-25,000 ft, but are made completely of ice crystals. They usually cover the sky as a thin veil or sheet of white. These clouds are responsible for the halos that occur around the sun or moon . The term "on cloud nine" (feeling of euphoria) is derived from the fact that the highest clouds are labeled category nine.

Middle level clouds

The mid level clouds 6,500-23,000 ft (2,000-7,000 m) typically have the prefix "alto" added to the two genera in this category. Altostratus clouds appear as a uniform bluish or gray sheet covering all, or large areas of the sky. The sun or moon may be totally covered or shine through very weakly. These clouds are complex as they are usually layered, with ice crystals at the higher, top layers, ice and snow in the middle, and water droplets in the lower layers. Altostratus clouds often yield precipitation . Altocumulus are elliptical, dense, fluffy balls. They are seen as singular units or as closely bunched groups in a clear sky. When the sun or moon shines through these clouds, one can sometimes see the "sun's rays" or corona.

Low level clouds

There are three genera in the low level (surface to 6,500 ft [2,000 m]). Stratus clouds are usually the lowest of the three genera. Stratus clouds blanket the sky and usually appear gray. They form when a large mass of air rises slowly and the water vapor condenses as the air becomes cooler, or when cool air moves in over an area close to ground level. These clouds often produce mist or drizzle. Fog is a stratus cloud at ground level. Cumulus clouds have flat bases, are vertically thick, and appear puffy. Inside a cumulus cloud are updrafts that create the cloud's appearance. They form when a column of warm air rises, expands, cools, and condenses. Cumulus clouds occur primarily in warm weather. They consist of water droplets and appear white because the sunlight reflects off the droplets. Thick clouds appear darker at the bottom because the sunlight is partially blocked. Cumulus clouds can develop into cumulonimbus clouds. Stratocumulus clouds are large, grayish masses, spread out in a puffy layer. Sometimes they appear as rolls. These clouds appear darker and heavier than the altocumulus cloud. They can transform into nimbostratus clouds.

Unusual clouds

Beside the basic cloud types, there are subgroups and some unusual cloud formations. Terms such as humulus (fair weather), and congestus (rainshower) are used to further describe the cumulus genus. Fractus (jagged), castellanus (castle shaped), and uncinus (hook shaped) are other descriptive terms used together with some basic cloud types. In mountainous regions, lenticular clouds are a common sight. They form only over mountain peaks and resemble a stack of different layers of cloud matter. Noctilucent clouds form only between sunset and sunrise, and are only seen in high latitude countries. Contrails (condensation trails) are artificial clouds formed from the engine exhaust of high altitude aircraft .

See also Weather forecasting; Weather modification.

Resources

books

day, john a., and vincent j. schaefer. peterson first guide toclouds and weather. boston, houghton mifflin co., 1998.

roth, charles e. the sky observer's guidebook. new york:prentice hall press, 1986.

rubin sr., louis d. and jim duncan. the weather wizard'scloud book. chapel hill: algonquin books of chapel hill, 1984.

shafer, vincent j., and john a. day. a field guide to the atmosphere. boston: houghton mifflin co., 1981.

Christine Minderovic