On a sunny day a wispy cloud can be a beautiful sight, but at other times a cloud can be a sign of storms to come. The different types of clouds reflect the weather conditions in which the clouds are formed.

How clouds form

The air outside always contains a certain amount of water vapor (that is, water in the form of gas), which is invisible. When air cools, some of the water vapor eventually condenses; that is, it forms tiny, visible water droplets. These droplets form around tiny particles, such as dust and grains of salt from ocean spray, that are in the air. If the droplets are on the ground they are called dew, if near the ground they are called fog or mist, and if up in the sky they are called clouds.

In the case of clouds, the droplets form when the moist air rises, since air cools as it rises. Some parts of the world are much more cloudy than others because the air rises more easily in those areas. Air can be forced to rise, as when it blows against a mountainside, or it can rise naturally through intense heating of the land. The areas around the equator are almost always cloudy during the middle part of the day. The air there is damp, and it rises because the land gets hot. When the land gets hot, the air above it gets hot, and hot air rises. In the great deserts, on the other hand, the air is so dry that clouds are rarely seen. The interiors of the vast continents of Asia and North America are not very cloudy because the winds blowing from the oceans have lost much of their moisture by the time they reach the middle of the continents. In some cases clouds are formed by the meeting of two masses of air, one cold and the other warm and moist. The warm air rises up over the cold air, causing it to cool and clouds to form.

Every cloud is made up of millions of tiny droplets of water floating together in the air. If the cloud is high enough, and therefore cold enough, the droplets are in the form of ice crystals. The droplets fall back to Earth in the form of precipitation.

Types of precipitation: rain, snow, and hail

For rain to form, the water drops in a cloud must become big and heavy enough to fall to Earth even though the air is cooling and rising. Sometimes the air becomes cool enough to produce snow clouds, which contain tiny ice particles that form when moisture in the cloud freezes. These ice particles then fall to Earth as snow. Hail is formed differently. Hail, or hailstones, are balls of ice that can be as small as a pea or as large as a tennis ball. These ice balls are created when rain droplets begin to fall from a cloud but are then carried upward by wind currents. Traveling higher in the sky, where it is colder, the droplets develop coatings of ice. They fall and are carried upward again and again, getting bigger and bigger as more and more ice accumulates. When they are too heavy to stay in the air, they fall to Earth as hail.

Types of clouds

Scientists classify clouds based mostly on their appearance. They have given clouds names that are combinations of several words: “cirrus,” or ice-crystal clouds; “cumulus,” or heaped-up clouds; and “stratus,” layer clouds. Two other words are used to describe clouds as well: “nimbus,” indicating rain, and “altus,” meaning “high” (but which scientists use to describe middle-level clouds).

There are ten main types of clouds. The highest clouds, which are found at heights of 3 to 8 miles (5 to 13 kilometers) above the Earth, are called cirrus, cirrocumulus, and cirrostratus. A cirrus cloud appears in delicate, featherlike detached bands, sometimes in tufts, and is usually white. Cirrocumulus clouds look like very small round balls or flakes. Cirrostratus clouds sometimes form tangled webs or thin whitish sheets.

The middle clouds, at 1 ¹/₄ to just above 4 miles (2 to 7 kilometers), are called altocumulus, altostratus, and nimbostratus. Altocumulus clouds are rounded puffs larger than cirrocumulus. Altostratus clouds cover the sky with a grayish veil through which the sun or moon may shine as a spot of pale light. Nimbostratus clouds are thick, dark, and shapeless.

Low clouds, at ground level to just over 1 mile above the Earth (up to 2 kilometers), are called stratocumulus, stratus, cumulus, and cumulonimbus. Stratocumulus clouds are large and lumpy, round or rolled-looking, and often cover the entire sky. Stratus clouds are generally dark and appear as streaks across the sky or as a gray layer hanging above the Earth. Cumulus clouds range in size from the small puffball-like forms to huge, dome-topped, thick piles of “woolpack” that often develop into turbulent thunderclouds. Cumulonimbus clouds look like large clumps of cumulus clouds with dark bases.

Precipitation generally falls only from altostratus, nimbostratus, and cumulonimbus clouds, all of which are found at middle and low heights. High clouds—cirrus, cirrocumulus, and cirrostratus—are composed of ice crystals and may produce snow, but the snow generally evaporates before it reaches the ground.