mid-latitude depression

An area of low atmospheric pressure occurring between 30° and 60°, shown on a weather map as a circular pattern of isobars with the lowest pressure at the centre. This low is some 1500-3000 km in diameter and is associated with the removal of air at height and the meeting of cold and warm air masses in the lower atmosphere. At the fronts between the air masses, a horizontal wave of warm air is enclosed on either side by cold air. The approach of the warm front is indicated by high cirrus cloud. With the approach of the front, the cloud thickens and lowers. Rain falls. As the warm sector passes over, skies clear and the temperature rises. The cold front is marked by heavier rain and a fall in temperature. See polar front.

The air behind the cold front moves more rapidly than the warm sector air and eventually pinches out the warm air, lifting it bodily from the ground to form an occlusion.

Mid-latitude depressions move at around 30 km per hour in summer and 50 km per hour in winter and last between four and seven days.