The Sun heats the ground, and the warm ground heats the air just above it. Warm air is lighter than cool air, so it rises. This is how many clouds begin.

As the air rises, it cools. The water vapor in the air turns into tiny drops of water. Those drops make a cloud. Water vapor is water in the air as a gas, and you cannot see it. You may have heard that a cloud is just water vapor. In fact, a cloud is made of drops of water, or bits of ice when it is very cold up high.

Warm air can hold more water vapor than cold air.

Clouds get their names from their shapes and from how high they float in the sky. Fog is a cloud that sits on the ground.

A thunderstorm brings lightning and thunder, usually with heavy rain. You see the flash first, because light travels much faster than sound. To find how far away the lightning is, count the seconds between the flash and the thunder. Then divide by 5 to get miles.

Rain falls when the drops in a cloud get too heavy to stay up. Snow needs cold air. If the air near the ground is too warm, snow melts on its way down and becomes rain.

Words to know

• vapor: water gas
• fog: ground cloud