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Issue Features
Discovery Magazine 12/1/1997


by  Wayne Jackson, M.A.

 Have you ever watched the clouds in the sky and wondered about them? What are they made of? How far away are they, and how do they move? Have you ever touched a cloud? Have you ever been “caught” in a cloud?

In 1803, an English scientist by the name of Luke Howard described three types of clouds based on their shape and height. He called them cirrus, cumulus, and stratus. Cirrus (sihr-uhs) clouds are thin, feathery clouds that are formed at high altitudes (heights above the Earth’s surface). The temperature at such altitudes is well below freezing, so cirrus clouds are made of ice crystals. These clouds are thin and wispy, because they form where the air is thin and the wind is strong. Sometimes they are called “mare’s tails,” because they look like a horse’s tail. They are fair-weather clouds.

Cumulus (kyoo-myuh-luhs) clouds are heaped, fluffy clouds, often with flat bases, that form as warm, moist air rises through the atmosphere. Warm air rises because it is lighter than cooler air. Cumulus clouds are found at all heights, and also generally indicate fair weather. If the push of air is strong, these clouds may extend many miles into the atmosphere, forming what are known as “thunderheads.” Another name for a thunderhead is cumulonimbus (kyoo-myuh-loh-nihm-buhs). When “nimbus” or “nimbo” is added to a cloud name, it means it can produce rain. A cumulonimbus cloud may bring violent weather, including thunder, lightning, hail, and tornadoes.

Stratus clouds spread out in a layer where a large body of damp air is lifted slowly into the atmosphere. Stratus clouds are not as tall as cumulus clouds, but they cover a wider area. Stratus clouds may block the Sun for many hours, or even days, because it take so long for them to pass.

Clouds can form at different altitudes. Cumulus and stratus clouds that develop at altitudes between one mile and four miles up are identified by the prefix alto-. For instance, a cumulus cloud at this altitude is called an altocumulus cloud. Cumulus and stratus clouds that develop at altitudes above four miles are identified by the prefix cirro-. A stratus cloud at this altitude is known as a cirrostratus cloud.

Clouds are made of water droplets or ice crystals. They can be white, or different shades of gray. When dense with moisture, they even can appear dark blue. The wind in the atmosphere moves the clouds; therefore, clouds are very important in predicting the weather.

Fog is a cloud on the ground. If you ever have been “caught” in fog, you know that sometimes you can’t see very far. If you have flown through a cumulus cloud in an airplane, you know that you can’t see through the cloud at all. How, then, does the airplane pilot know how to fly through a cloud? Fortunately, he has instruments onboard that help him know where to go.

During an electrical storm, we often are frightened by thunder and lightening—and rightly so! Knowledgeable adults warn us to stay inside during such storms. The thunder won’t hurt us, but lightning certainly can. Lightning occurs when an electrical charge in the clouds is attracted to something on the ground.

Hailstones form in cumulonimbus clouds when rising wind currents (updrafts) force raindrops into freezing air. The raindrops turn to ice, fall, pick up more water, and then rise again to make bigger hailstones. The stronger the updrafts, the bigger the hailstones can grow. Eventually, the hailstones can fall to the ground as anything from pea-sized pellets to baseball-sized chunks of ice. Hail can do tremendous damage to crops, and has been known to break windows and damage other property as well.

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