front

The boundaries between two air masses of different temperature and humidity. A warm front occurs when warm air displaces cold air; if cold air replaces warm air, it is a cold front.

In meteorology, the boundary between two air masses of different temperature or humidity. A cold front marks the line of advance of a cold air mass from below, as it displaces a warm air mass; a warm front marks the advance of a warm air mass as it rises up over a cold one. Frontal systems define the weather of the mid-latitudes, where warm tropical air is continually meeting cold air from the poles.

Warm air, being lighter, tends to rise above the cold; its moisture is carried upwards and usually falls as rain or snow, hence the changeable weather conditions at fronts. Fronts are rarely stable and move with the air mass. An occluded front is a composite form, where a cold front catches up with a warm front and merges with it.