Weather is what the air outside is doing, like rain, snow, wind or sunshine. It changes from day to day, and even from hour to hour. Climate is different. Climate is the usual weather in a place over many years. A weather scientist is called a meteorologist. Meteorologists make forecasts that tell what the weather will be.

The Sun heats the ground. The warm ground heats the air just above it. Warm air is lighter than cool air, so it rises.

Air has weight. The weight of all the air above us pressing down is called air pressure. Wind is air that is moving. It blows from places with high air pressure toward places with low air pressure. High-pressure areas usually bring calm, clear weather. Low-pressure areas can be stormy.

Here is how a cloud is made. The Sun's warmth turns water into vapor. This is called evaporation. As the air rises, it cools. The vapor turns into tiny drops. This is called condensation, and the drops make a cloud. Humidity is the amount of water vapor in the air. Humid air feels muggy.

Rain falls when the drops in a cloud get too heavy to stay up in the sky. Snow is made of tiny ice crystals that stick together. Fog is a cloud that sits on the ground. Next time it is foggy, remember that you are walking inside a cloud.

A thunderstorm brings lightning and thunder, usually with heavy rain, and sometimes with hail. Lightning is a giant spark of electricity. Thunder is the sound it makes. Hail grows inside the storm. Rising air keeps a hailstone high up while more ice freezes onto it. At last it is too heavy, and it falls.

You see lightning before you hear thunder, because light travels much faster than sound. To find how far away a flash was, count the seconds until the thunder and divide by 5 to get miles.

Some storms spin. A tornado is a spinning column of air. It reaches from a storm cloud down to the ground. A hurricane is a huge spinning storm. It forms over warm ocean water.

For safety, remember this rule: when thunder roars, go indoors. A building or a car with a roof keeps you safe from lightning.

Words to know

• meteorologist: weather scientist
• condensation: vapor to drops
• humidity: wet air
• pressure: air weight

The short version

Weather is what the air outside is doing, like rain, snow, wind or sunshine. The Sun heats the ground and the air above it, and warm air rises. As it rises it cools, and its water vapor turns into tiny drops that make clouds. Rain falls when the drops get too heavy to stay up.

Quick facts

• Weather changes from day to day, and even from hour to hour.
• Wind blows from places with high air pressure toward places with low air pressure.
• Lightning is a giant spark of electricity, and thunder is the sound it makes.
• When thunder roars, go indoors: a building or a car with a roof keeps you safe from lightning.