In 1802, Luke Howard, a London pharmacist and amateur meteorologist, first proposed sorting clouds into types. His book The Modifications of Clouds named the cloud shapes he studied, and we largely use the same system today. Howard's system had three main types, stratus, cumulus and cirrus. Stratus clouds are low, gray clouds that can cover the whole sky like a blanket, cumulus clouds are puffy, lumpy clouds, and cirrus clouds are thin, wispy clouds high in the sky. The World Meteorological Organization later extended the system to 10 main groups called genera, in three levels: low, medium and high.

Clouds are made of tiny drops of water or tiny bits of ice. They are not just water vapor, because water vapor is an invisible gas. Water vapor is the gas form of water, and it is in the air all around us. The water droplets in a cloud are only about a hundredth of a millimeter across, but a cloud holds a huge number of them. It takes millions of cloud droplets to make a single raindrop.

Clouds form when air rises and cools. They form when water vapor in the air turns into tiny drops of liquid water, and cloud drops form around tiny specks such as dust, pollen or salt. The Sun heats the ground, the warm air above it rises, and cumulus clouds can form. Air pushed up a mountain cools and can make clouds. At a weather front, warm air rises over cold air, and clouds form.

You might think a cloud weighs nothing, but a typical cumulus cloud might weigh more than one million pounds, yet a plane flies right through it. Its water is spread out as tiny droplets that float. Cloud droplets are too small to fall as rain. 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.

Fog is essentially a cloud at ground level, and it cuts visibility to less than 1,000 meters (about 3,300 feet). Stratus clouds can look like fog, and they make only drizzle or light snow, if they make anything. Stratocumulus clouds are low, lumpy and gray, and they give only light rain. They are often mistaken for rain clouds, but it is rare to get more than the lightest drizzle from them.

You can use your hand to tell some clouds apart. To tell stratocumulus from altocumulus, point your hand toward the cloud. A stratocumulus cloud looks about the size of your fist. Altocumulus clouds are mid-level clouds that form in rounded clumps. In the middle latitudes, high clouds are found roughly 5 to 13 kilometers (about 16,000 to 43,000 feet) up, and cirrus clouds are made of ice crystals.

Cumulus clouds often show up on sunny days, and some grow into storm clouds. A thunderstorm needs three things: moisture, rising unstable air, and something to give the air a push. A cumulonimbus cloud, which is a thundercloud, 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.

Thunder is the sound that lightning makes. It is not possible to have thunder without lightning. People who see lightning but hear no thunder are too far away to hear it. 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. When you hear thunder, go indoors, into a building or a car with a roof.

Words to know

• meteorologist: weather scientist
• genera: main groups
• visibility: seeing distance
• anvil: flat-topped shape