Cold air flows outwards from the poles, while warm subtropical air moves polewards. The cold and warm air meet along a line called the polar front. Because of the great difference in temperature, the cold and warm air do not mix. Instead, warm air flows into bends in the polar front and cold air flows in behind it. This sets up a rotating low pressure air system, called depression or cyclone, which has warm, light air at its center.
The front edge of the warm air is called the warm front. Ahead of the warm front, the warm air flows upwards over the dense, cold air. As it rises, a blanket of cloud forms and rain starts to fall from it. The advancing edge of the cold air, behind the warm air, is called the cold front. Here, cold air pushes under the warm air and thunderclouds often form as the warm air rises. Depressions, therefore, bring stormy, unsettled weather as they move across the land.