An area of low atmospheric pressure occurring between 30° and 60°. This low is some 1 500–3 000 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. The cloud then thickens and lowers, and 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. The warm front advances at 20 to 30 miles per hour, whilst the cold front can move forward more quickly at 40 to 50 miles per hour. In consequence, the cold front eventually pinches out the warm air, lifting it bodily from the ground to form an occlusion. See R. Barry and R. Chorley (2003).