The water on Earth is constantly moving in the water cycle, driven by energy from the Sun and by gravity. The Sun heats oceans, lakes and rivers, and water evaporates into water vapour. Plants also release water vapour from their leaves in transpiration. As the warm, moist air rises it cools, and the vapour condenses into tiny droplets that form clouds.
When droplets join and get heavy, they fall as precipitation: rain, snow, sleet or hail. On the ground, water flows over the surface as run-off into rivers, or soaks into the soil and rocks as groundwater. Eventually it returns to the sea, and the cycle repeats. The total amount of water stays almost the same; it just changes state and place.
Worked example
Why do clouds form as air rises?
- Rising air expands and cools.
- Cool air cannot hold as much water vapour.
Answer:The vapour condenses into droplets, forming clouds.
Key words
- evaporation
- liquid water turning into vapour
- condensation
- vapour cooling into liquid droplets
- precipitation
- water falling as rain, snow, sleet or hail
- transpiration
- water vapour released from plant leaves
New questions every time.
NGSS-referenced: MS-ESS2-4