Fronts occur at the boundaries of converging air masses which come together from different parts of the world. Since air masses usually have different temperatures, they cannot mix together immediately owing to their different densities. Instead, the lighter, warmer air mass begins to rise above the cooler, denser one.
Fronts are usually associated with depressions, regions of low pressure centred on the rising air which develop as a result of the Earth's rotation. As the sector of warm air is forced to rise, the cold air begins to engulf it. The leading edge of the warm air is marked by the warm front. The cold front marks the rear edge of the warm air and the leading edge of the ensuing cold air. When the warm air is completely uplifted off the ground, this may be marked on a synoptic chart by an occluded front.
Fronts are accompanied by clouds of all types, and very often by precipitation. Precipitation is usually heavier although less prolonged at cold fronts than at warm fronts, since the uplift of warm air there is more vigorous due to the undercutting of cold air, resulting in increased atmospheric instability.