Clouds

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