Clouds are made of water—thousands of gallons of water, floating high in the air. It’s easier to believe this when you know that cloud water takes the form of tiny droplets. The droplets are so tiny that you couldn’t see one if it was separated from all the others. If all these water droplets in a cloud meet a mass of warm air they evaporate – and the cloud disappears! This is why clouds are constantly changing shape.
Sometimes the water droplets join together around tiny pieces of dust in the air. These drops get bigger and bigger as more droplets collect. When they become too heavy to float, they fall.
There are three main kinds of clouds. “Cumulus” refers to the small puffballs or great wooly-looking clouds that are flat on the bottom. “Stratus” are low clouds, usually streaky or without much shape. And “cirrus” are light feathery clouds, like the ones in the photo. Sometimes cirrus clouds are so high, where the air is very cold, that the whole cloud is made of ice.
Adding “nimbus” to any of these names changes it to mean a rain cloud. Tall white cottony rain clouds are called “cumulonimbus,” or thunderheads. They often bring thunderstorms. Flat gray rain clouds are called “nimbostratus.” They usually bring only rain.
Snow, sleet, and hail also fall from clouds. Snow and sleet fall only on cold winter days. Hailstones can fall even on a warm summer day.
In the summer, small cumulus clouds that appear in the morning often turn into dark cumulonimbus clouds during the day.
Beside the basic cloud types, there are subgroups and some unusual cloud formations.
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.