Earth’s water cycle: Why does it rain?
Rain is defined as liquid precipitation. It is formed high above the ground in clouds by water vapour coming together into large droplets that become too heavy for the air to support.
Gatherings of condensed water vapour are called clouds, and this is where rain comes from. Although a large amount of water is held in each cloud, rain does not fall all at once from the cloud because the water droplets grow at different speeds, with the fastest growing droplets becoming heavier quicker and falling first, the slower growing ones falling after.
Although scientists are not sure if there is water on other planets, the phenomenon of rain has occasionally been recorded. This rain involves other liquids, such as methane which falls on Titan, Saturn’s moon, and sulphuric acid which falls on Venus.
How it rains
1. Water evaporates
Water on the Earth’s surface is heated by the Sun, and small water droplets evaporate into the air.
2. Water vapour travels
The heat from the Sun pulls these water vapour droplets upwards into the atmosphere.
3. Clouds form
When the water vapour droplets reach a certain level, condensation starts to occur, forming clouds. This is due either to an increase in humidity or a drop in temperature in the atmosphere.
4. Rain forms
As more water vapour droplets gather within the cloud, they start to merge and become larger. Eventually they will become too heavy to be held in the atmosphere and gravity will start to pull them back towards Earth.
5. Rain falls
This water falling back to Earth is called rain. Sometimes it will not make it back to Earth, due to the air not being humid enough, and in this case, water evaporates back into the atmosphere. If the temperature of the air is lower than normal, the water will freeze and fall as snow or hail instead of rain.
6. Water returns to the Earth
If the water that falls is not caught by plants or animals, water hits the ground and, if not evaporated from the land, it will start to move back towards the rivers, streams and the sea – where the process begins again.
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