Animation of the development of a low pressure system as it crosses the British Isles. A low, or depression, is a region of relatively low atmospheric pressure that forms along a front boundary. Initially, warmer air from the tropics is to the south moving northwards a warm front, and cooler air from the poles is to the north, moving south a cold front. This semi_stable arrangement is called a stationary front. The direction of movement of the fronts is indicated by the blue triangles cold and red semicircles warm on a weather map, and by arrows here. The Coriolis effect causes the direction of movement, with warm air pushing northeastwards and cold air southwestwards. Cold air moves faster than warm as it is denser, and the cold front sweeps south, undercutting the warm air. These factors contribute to the development of a low pressure area at the centre of the system. The circular lines show atmospheric pressure at 4 millibar intervals. The point of view then zooms to the ground, looking north from within the shrinking wedge of warm air. On the ground, high cirrus clouds precede the passage of the warm front, with stratus clouds over the front boundary. The warm air between the fronts has scattered clouds, as the warm air rises. The cold front moving rapidly from the west undercuts the warm air, forcing it up over the cold air, and this rapid upward passage causes the formation of tall clouds that bring heavy rain and thunderstorms. The faster_moving cold front catches the warm front, pushing under the front boundary and forming an occluded front purple line, with associated cloud and rain. The occluded front continues to move eastward, until it has passed, leaving cold high pressure air and clear skies behind.

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    1920X1080

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    000:28.000

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