Weather is a word to describe what is happening in the air: rain or snow, hot or cold, cloudy or clear, windy or still. The Sun’s heat is vital to our weather. It makes some lands warmer than others. The land warms the air above it. Warm air rises, so cooler air flows in to replace it, producing winds.

Clouds and rain

Clouds consist of millions of tiny water droplets or ice particles. When water vapour in the air condenses, water droplets form around dust particles. If the temperature falls below freezing, the droplets turn to ice. They float in the air until they become too heavy. They then fall (precipitate) as rain, or snow if the air below is freezing.

Snow

High above the Earth’s surface, water vapour freezes. It forms delicate crystals of ice. These stick together with others to form snowflakes. When the snowflakes become too heavy, they fall to the ground.

Fog

Fog or mist (a thin fog) is cloud that hugs the ground. It is often foggy when moist air cools at night, causing water droplets to form in the air. Fog also forms when moist air is forced to rise over a hill.

Weather forecasting

Without rain, our crops would die. Too much rain can cause dangerous floods. It is very useful to be able to predict what the weather has in store for us. To do this, forecasters collect information about the weather from satellites in space and from weather stations. They then work out what weather we can expect in the next few days.