22. Why does it rain?

• The air we breath contains up to 4% water vapour.

• Warm air can hold more water vapour than cold air.

• Air at 30⁰C can hold three times more water than air at 10⁰C.

• The amount of water in the air is called humidity.

• When the air becomes saturated with water vapour, 100% humidity, it can't take any more and the excess vapour condenses back into liquid water as cloud droplets and clouds form.

• Cloud droplets are about 0.04 mm in diameter and although gravity tries to pull them to the ground, air currents keep them floating in the air.

• Cloud droplets need to get bigger to from rain drops which fall to the ground.

• Rain drops are normally between 0.5 and 4.0 mm in diameter.

There are two main ways cloud droplets form into rain droplets:

• they bump into each other and join or coalescence; and

• water vapour condenses around ice crystals.

Cooling the air reduces it's ability to hold water vapour and clouds form. There are three main ways the air is cooled: convection; orographic and frontal.

Convective cooling The sun warms the ground. The air next to the ground warms by convection (contact). The warm air expands, becomes less dense than the air above and rises. As you go up through the atmosphere it gets cooler at a rate of 10C per 100 metres or 100C per kilometre. So as a bubble of warm air rises, it starts to cool and cool air can't hold as much water as warm air so eventually it becomes saturated with water. At this altitude, water vapour starts to condense into liquid cloud droplets. This is why many clouds have flat bottoms.
Mountain (orographic) cooling Wind, air which is moving, cannot go through mountains, it has to go over them. As the air rises, it cools and clouds may form.
Frontal cooling When a cool mass of air moves in from a cold area such as the Southern Ocean, it is denser than the warmer air in front. It pushes the warmer air up, this air cools and clouds may form.

Pour very cold water into a glass. What happens on the outside of the glass? Explain why.

Water cycle	Different clouds

Riverina Environmental Education Centre