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CGS Ground School Meteorology Fronts © Crown copyright 2012. No part of this presentation may be reproduced without the permission of the issuing authority. The views expressed in this presentation do not necessarily reflect the views or policy of the MOD. Fronts A frontal surface is the boundary between two adjacent airmasses of different temperatures. The line where the frontal surface meets the ground is called a front. The front which chiefly affects the UK is the polar front which marks the boundary between the polar and tropical airmasses. There are different kinds of polar fronts, the following slides show the main fronts that affect the UK. Stationary front A front making no progress in relation to the ground. The frontal surface slopes due to the warm air being less dense than the cool air. Synoptic Chart Symbol: Warm air Cold air Warm front A front at which warm air is replacing cool air. The frontal gradient is typically between 1:100 and 1:150. Surface friction reduces the speed of a warm front to approx ⅔ of the speed of the surrounding airmass. Synoptic Chart Symbol: Frontal movement Warm air Cold air Cold front A front at which cool air is replacing warm air. The frontal gradient is typically around 1:50. Synoptic Chart Symbol: Surface friction may slow the progress of the front, however the cool airmass continues to move at the speed of the surrounding airmass. This causes the formation of a "nose" approx 2000 ft high. This nose creates a very unstable situation, which will periodically collapse giving a line squall. This also explains the erratic approach of a cold front. Frontal movement Cold air Warm air Occluded front A front formed when a cold front catches up with a warm front and either rides up it (warm occlusion) Frontal movement Warm air Cool air Cold air Occluded front A front formed when a cold front catches up with a warm front and either rides up it (warm occlusion) or pushes beneath it (cold occlusion). Synoptic Chart Symbol: Frontal movement Warm air Cold air Cool air Development of fronts The polar front separates the cool polar air from the warmer tropical air. The warm air tends to flow eastward. Air in the polar region is at high pressure, the cool air flowing away from the polar region therefore tends to flow westward. To understand the reason for this deflection see the presentation ‘wind’. Development of fronts In an area where the warm air flows towards the cold air, it will be forced to rise. This causes the local pressure to fall and a wave starts to develop from the warm air into the cool air. Pressure within the vicinity of the wave reduces even further. L Development of fronts As the wave increases in size the pressure drops further, becoming lowest at the wave crest. The polar front has now become a warm front to the east and a cold front to the west. L Development of fronts As the wave increases in size the pressure drops further, becoming lowest at the wave crest. The polar front has now become a warm front to the east and a cold front to the west. As the pressure in the centre continues to fall, a system of closed isobars develops around the depression. The polar air will flow in towards the low pressure region in an attempt to equalise the pressure. L Development of fronts The air begins to flow around the depression in an anti-clockwise direction. As the depression continues to deepen, it moves eastward and the faster moving cold front starts to catch up with the slower moving warm front. L Development of fronts The air begins to flow around the depression in an anti-clockwise direction. As the depression continues to deepen, it moves eastward and the faster moving cold front starts to catch up with the slower moving warm front. L Development of fronts The air begins to flow around the depression in an anti-clockwise direction. As the depression continues to deepen, it moves eastward and the faster moving cold front starts to catch up with the slower moving warm front. Development of fronts Eventually, in the centre of the depression, the cold front catches up with the warm front and an occluded front is formed. Initially the point of occlusion remains in the area of lowest pressure. However the pressure at the point of the occlusion rises as the polar air continues to flow in. Development of fronts This causes the point of the occlusion to move southward, away from the centre of low pressure. Development of fronts This causes the point of the occlusion to move southward, away from the centre of low pressure. As the warm sector becomes smaller so the occlusion begins to degenerate and the cycle comes to an end. Wind and pressure changes As any front passes there will be a change in wind direction and surface pressure. The warm front sequence below could represent any type of front. Consider an observer at point A. As the front approaches the pressure reduces. As the front passes the wind veers. Behind the front the pressure 964 968 increases. 972 976 980 A Weather changes Passage As well asofthe a warm cold windfront direction front. and air pressure changing, the most obvious sign of the passage of a frontal system Consider an observer at point A is a change in cloud types and associated precipitation. At the front theofofcloud isobserver almost at ground level. The Behind In the cloud approach passage cold thecontinues sector, front the the the the front rain cold to lower continues isfront marked (cirro is can marked for stratus by see up low clear by to& cloud 50 aalto skies build miles. and stratus). with up of The passage ofpersistent the until frontpossibly is marked byband the wind veering. Rain upper cumulus heavy cloud becomes cloud rain. clouds. then (Cirrus lifts and eventually over a wide Cirrostratus) the skies (up clear and to 200 and medium nm In the (Nimbostratus) warm sector thelevels cloud low (stratus) and may ahead cloud cumulus Gradually Where of the clouds the increasing frontal front) begin nose and just to the collapses, form. ahead cloud ofremains upper of lowers itthe will cloud front. also further form be marked be accompanied by light drizzle. (nimbostratus Followed These approximately by a line may squall. bycontinue a& relatively 400nm stratus), toahead develop narrow of the band vertically front. of rain. to give cumulonimbus clouds with associated rain. Cold sector 40,000ft 30,000ft 20,000ft Cu 10,000ft A Weather changes Passage of an occluded front. rain bandofisan 100-200 miles wide on a warm occlusion The passage occluded front is generally similar to a and 50-100 miles widecloud on a cold occlusion. warm front with upper gradually being replaced by A coldcloud. occlusion may also have embedded cumulonimbus lower clouds at the front. 40,000ft Cool sector 30,000ft Cb Cu 20,000ft 10,000ft THE END Any Questions?