Rain is formed during a process which is usually known as the water cycle. The water cycle involves a number of steps, including evaporation, cloud formation, precipitation (rain), relocation, and then evaporation again.

Evaporation occurs when liquid water is vaporised into water vapour, allowing it to become a part of the atmosphere. Heat from the sun acts on the water molecules in liquid water, increasing their energy. These particles then ‘break away’ from the liquid, becoming airborne. When there is high humidity it simply means that there is a high concentration of water molecules in the air: a lot of evaporation has occurred.

When the amount of water vapour in the air reaches a level where no more water can evaporate, we say that the air is fully saturated, or that the humidity is at 100%. If the conditions then change so that the air can hold less water vapour (for example, if it gets colder or the barometric pressure decreases), then some of it will begin to condense and form liquid water again. To begin with, only tiny particles of water will be formed which are too light for gravity to pull them back to the ground. If this happens near ground level, fog or mist will be formed. However, if it happens in the upper atmosphere, we will see collections of these water particles as clouds.

Inside a cloud, the tiny water particles move around, bumping into each other. When they hit each other, they can combine to form a larger droplet. Once this has happened enough times, a droplet of water which is heavy enough to fall to the ground is formed. The form of this fall – known as precipitation – depends on atmospheric conditions. It may be snow, rain, sleet, hail, etc. Once precipitation, or rainfall, has occurred, the liquid water will move around the earth’s surface, usually towards the ocean, until it is evaporated again and the cycle restarts.