Evaporation occurs when solar energy from the sun heats water in rivers or lakes or the ocean. As water molecules get hotter, they begin to move faster in the liquid. They collide more often with each other and, gradually, some water molecules move fast enough to break away from the others. These water molecules escape into the air or 'evaporate'. By this stage, the water has changed from being a liquid into a gas. It has turned into water vapour or steam. This rises into the atmosphere.
The sun also contributes to evaporation by unevenly heating the Earth's surface. Warm air rises while cool air, which is denser and heavier, falls. The movement of the air becomes wind, which helps evaporation. How quickly evaporation occurs depends on several factors:
- Wind speed: The faster the wind blows, the more water evaporates
- Temperature: The hotter the temperature, the more water is evaporated
- Humidity: The greater the humidity, or the level of moisture in the air, the slower the rate of evaporation. When the air is already full of tiny water droplets, there is less space left for water to evaporate.