What are clouds?

Clouds are formed when moist air is cooled and rises. As it does this, it expands and eventually becomes saturated and the water vapour condenses in water droplets, which are similar to those found in fog.

How does air rise to form clouds?

There are 4 main ways in which it does this:

• When heated air at the earth’s surface rises quickly in the form of thermal currents (convection). This will produce a cumulus type cloud.
• When warm moist air is undercut by cold air, it produces a slow but widespread ascent (this barrier between the warm and cold air is known as a ‘front’).
• A frictional effect of the earth’s surface produces an upward motion. This and the previous point tends to produce layered type clouds.
• Air is forced to rise over a barrier e.g. mountain range or hills. This will produce either cumulus type or layered type cloud depending on the state of the atmosphere.