Weather and the water cycle

The Sun warms the water of rivers, lakes and seas, and part of it turns into vapour (evaporates) and rises. High up the air is cold, so the vapour turns into tiny water droplets (condensation) and clouds form. When the drops get heavy they fall as rain or snow. Water flows into streams and rivers and returns to seas and lakes. In this way water keeps circulating. The amount of water vapour in the air is called humidity. White, fluffy heaps of cloud usually give no rain; thick dark clouds bring rain or snow. Rainfall is measured in millimetres with a rain gauge.