Air masses are large areas of air which have the same characteristics throughout. They may be warm or cold, depending upon where they originated, ie tropical or polar. They are further classified as either continental or maritime. When two different air masses meet they form a boundary which is known as a front. Most fronts are in motion, but if a front is stationary, it is referred to as quasi-stationary. When the approaching air is colder than the present air the boundary will be referred to as a cold front, whereas if the approaching air is warmer than the present, the boundary is referred to as a warm front.

The boundary between polar and tropical air masses is known as a polar front. Since polar air is denser than tropical air, the latter overrides the former. The surface of the front is therefore not vertical but inclined towards the colder. The slope of a cold front is approximately 1:50, whilst that of a warm front is approximately 1:100.