Look up on a cloudy day. Clouds are made of tiny drops of water or tiny bits of ice. They look white because sunlight is white. Rain and snow fall out of clouds. So does hail. Hail is solid balls of ice that fall from clouds.

Clouds are sorted into high, middle and low groups. The groups depend on how high the clouds are in the sky. Cumulus clouds are puffy. Cirrus clouds are thin, wispy clouds high in the sky. They are made of tiny bits of ice.

Altostratus clouds are gray sheets that cover the sky. They make the Sun look fuzzy. Stratus clouds are low, gray clouds. They can cover the whole sky like a blanket. Stratocumulus clouds are low, lumpy and gray. They give only light rain.

Cumulus clouds often show up on sunny days. Some grow into storm clouds. A cumulonimbus cloud is a thundercloud. It can be about 10 kilometers tall. That is 33,000 feet or 6 miles. Nimbostratus clouds are dark gray clouds. They bring steady rain or snow.

Some clouds look odd. A sky full of small cirrocumulus clouds can look like fish scales. It is called a mackerel sky. Lenticular clouds are lens-shaped. They often look like flying saucers. The white streaks behind high-flying jets are clouds called contrails.

Thunder is the sound that lightning makes. You see lightning before you hear its thunder. That is because light travels faster than sound. Here is what to do about it. When you hear thunder, go indoors. Go into a building or a car with a roof.

Words to know

• cumulus: puffy cloud
• hail: balls of ice
• contrails: jet clouds
• cumulonimbus: thundercloud