Clouds are made of tiny drops of water or tiny bits of ice, and they look white because sunlight is white. Water vapor is the gas form of water, and it is in the air all around us. Clouds form when water vapor in the air turns into tiny drops of liquid water. Clouds form when air rises and cools, and cloud drops form around tiny specks such as dust, pollen or salt.

Air can be lifted in several ways. The Sun heats the ground, the warm air above it rises, and cumulus clouds can form. Cumulus clouds are puffy, lumpy clouds that often show up on sunny days, and some grow into storm clouds. Air pushed up a mountain cools and can make clouds. At a weather front, warm air rises over cold air, and clouds form. Each of these is a different way of doing the same job, which is raising the air high enough that it can cool and make a cloud.

Clouds are sorted into high, middle and low groups by how high they are in the sky, and each group has its own kinds of cloud. In the middle latitudes, high clouds are found roughly 5 to 13 kilometers (about 16,000 to 43,000 feet) up. Cirrus clouds are thin, wispy clouds high in the sky, and they are made of ice crystals. Altocumulus clouds are mid-level clouds that form in rounded clumps, and stratus clouds are low, gray clouds that can cover the whole sky like a blanket.

A cloud does not drop its water right away, and it helps to follow the water as it leaves the cloud. Cloud droplets are too small to fall as rain, so they float in the air, and rain falls when cloud droplets grow big and heavy enough to leave the cloud. Rain and snow fall out of clouds, and hail is solid balls of ice that fall from clouds. Nimbostratus clouds are dark gray clouds that bring steady rain or snow.

Some clouds grow into storms, and it is worth seeing how that happens. Cumulonimbus clouds are thunderclouds. A thunderstorm needs three things: moisture, rising unstable air, and something to give the air a push. A cumulonimbus cloud can be about 10 kilometers (33,000 feet or 6 miles) tall, and a tall thundercloud can flatten at the top into the shape of an anvil. Lightning heats the air around it to about five times hotter than the surface of the Sun.

The white streaks behind high-flying jets are clouds called contrails. Contrails form when water vapor from a jet's exhaust and the air condenses and freezes into a cloud. Some clouds look odd, and lenticular clouds are lens-shaped and often look like flying saucers. A sky full of small cirrocumulus clouds can look like fish scales, so it is called a mackerel sky. On Jupiter, the stripes and swirls are clouds, made of ammonia and other chemicals.

There is one last thing to learn about storms, and it matters most. Thunder is the sound that lightning makes. You see lightning before you hear its thunder, because light travels faster than sound. To find how far away lightning is, count the seconds between the flash and the thunder and divide by 5 to get miles. These facts matter most when a storm is near, so it is worth knowing them well. When you hear thunder, go indoors, into a building or a car with a roof.

Words to know

• vapor: water as gas
• front: weather boundary
• latitudes: bands of Earth
• ammonia: a chemical