Hydrology is studying the movement, distribution (how the water is spread out across Earth) and the quality of water on Earth. It mainly focuses on the water cycle, which is explained below:

The water cycle

The water cycle explains how water ends up in our rivers and lakes:

1. The water evaporates (turns from a liquid to a gas) and rises into the atmosphere.
2. In the atmosphere, the water cools and condenses, which form clouds.
3. When the cloud is heavy enough, either rain, snow, hail or sleet falls (which one it is depends on the temperature of the ground below.) This is called percipitation.