World Book at NASA for Students
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Clouds are large amounts of tiny water droplets or tiny pieces of ice that float in the air. They may be white, gray, or black. Clouds play an important part in the Earth's weather. The water they bring as rain and snow is needed by all forms of life. Clouds can also bring dangerous weather.
Image to right: Solar radiation heats the damp ground and the warm air next to it. The warm air becomes lighter and is carried upward. As it rises, it becomes cooler. If the air is moist, it can form clouds. Credit: World Book illustration by Herb Herrick
Kinds of clouds
There are many different kinds of clouds. Scientists gave them names that tell us what they look like. Clouds that look like layers or sheets are called stratus clouds. Cumulus clouds are piled-up masses of white clouds. Cirrus clouds are curly white clouds. These are the most common kinds of clouds.
How clouds form
The heat of the sun warms lakes, oceans, and rivers. As these waters warm, some of the water evaporates -- it changes from liquid water to a gas called water vapor. The water vapor rises into the air and cools as it rises. As it cools, some of the water vapor begins to change back into tiny drops of water. If it is cold enough, it turns into tiny pieces of ice. Those drops of water or pieces of ice come together to form clouds.
Rain or snow comes from clouds. This happens when the water droplets inside a cloud freeze onto the pieces of ice. The pieces of ice grow bigger and bigger until the cloud can no longer hold them. So the ice falls out of the cloud and falls to Earth as a snowflake. However, if it enters a layer of air warm enough to melt the snowflake, it falls to Earth as a raindrop.
People who forecast, or predict, the weather study clouds carefully. Certain kinds of clouds often appear before a storm. For example, some kinds of clouds appear when a warm front -- a mass of warm air -- comes through an area. Warm fronts move clouds in a certain order over many days. As these clouds move through an area, they usually bring rain.
Clouds also help heat and cool the Earth. Cloudy days are usually cooler than clear days because clouds reflect, or send back, sunlight into space. Then the sunlight does not get a chance to heat the Earth. The opposite of this is true at night. The Earth gives off heat toward space. This causes the ground to cool off. Clouds stop much of this heat and send it back toward the ground. So cloudy nights are usually warmer than clear nights. The heat is trapped between the cloud and the ground.
How to cite this article: To cite this article, World Book recommends the following format: "Clouds." The World Book Student Discovery Encyclopedia. Chicago: World Book, Inc., 2005.
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