Depressions, sometimes called mid-latitude cyclones, are areas of low pressure located between 30° and 60° latitude. Depressions develop when warm air from the sub-tropics meets cold air from the polar regions. There is a favourite meeting place in the mid-Atlantic for cold polar air and warm sub-tropical air. Depressions usually have well defined warm and cold fronts, as the warm air is forced to rise above the cold air. Fronts and depressions have a birth, lifetime and death; and according to the stage at which they are encountered, so does the weather intensity vary.
A depression appears on a synoptic (weather) chart as a set of closed curved isobars with winds circulating anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere due to the rotation of the Earth. The warm and cold fronts associated with depressions bring with them characteristically unsettled weather. Depressions vary from between 200 and 2,000 miles in diameter; they may be deep when pressure at their centre is very low and the isobars are tightly packed, or shallow when less well developed.
A depression develops like the propagation of a wave in water. Initially, a uniform boundary or front exists between cold air pushing southwards and warm air pushing northwards (in the Northern Hemisphere). A wave-shaped distortion may appear on the front, and a small low-pressure centre develops at the crest of the wave. In the immediately surrounding area the pressure begins to fall. A disturbance of this kind is called a wave depression. As the "wave" develops, a warm sector of air forms bounded by the warm and cold fronts, which begins to tie over the engulfing cold air. Both the warm and cold fronts originate from the centre of the depression. On the ground, sudden changes in the wind direction may be experienced when fronts pass by.
Wave depressions can grow off the tail ends of primary cold fronts. The depression so formed is then called a secondary depression. New centres may also develop from occluded fronts within the primary depression. The secondary system can then become the main system, and the primary occluded front becomes caught up in the developing circulation, effectively becoming a third front.