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. SENDING A FIRST SOLO DISCUSS THE FOLLOWING 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 SENDING A FIRST SOLO DISCUSS THE FOLLOWING 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 SENDING A FIRST SOLO DISCUSS THE FOLLOWING 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 SENDING A FIRST SOLO DISCUSS THE FOLLOWING 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 SENDING A FIRST SOLO DISCUSS THE FOLLOWING 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) Synoptic Chart Symbol: or pushes beneath it (cold occlusion). Frontal movement Warm air SENDING A FIRST SOLO DISCUSS THE FOLLOWING 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’. SENDING A FIRST SOLO DISCUSS THE FOLLOWING

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. SENDING A FIRST SOLO DISCUSS THE FOLLOWING 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. SENDING A FIRST SOLO DISCUSS THE FOLLOWING 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. SENDING A FIRST SOLO DISCUSS THE FOLLOWING 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. SENDING A FIRST SOLO DISCUSS THE FOLLOWING 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 SENDING A FIRST SOLO DISCUSS THE FOLLOWING

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. SENDING A FIRST SOLO DISCUSS THE FOLLOWING

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. SENDING A FIRST SOLO DISCUSS THE FOLLOWING

Development of fronts This causes the point of the occlusion to move southward, away from the centre of low pressure. SENDING A FIRST SOLO DISCUSS THE FOLLOWING

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. SENDING A FIRST SOLO DISCUSS THE FOLLOWING

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. 964 968 972 976 980 SENDING A FIRST SOLO DISCUSS THE FOLLOWING Behind the front the pressure increases. A

Weather changes A Passage of a cold front Passage of a warm front. As well as the wind direction and air pressure changing, the most obvious sign of the passage of a frontal system is a change in cloud types and associated precipitation. Consider an observer at point A In the cold sector, the observer can see clear skies with cumulus clouds. Gradually increasing levels of upper cloud form approximately 400nm ahead of the front. At the front the cloud is almost at ground level. The passage of the front is marked by the wind veering. In the warm sector the cloud remains low (stratus) and may be accompanied by light drizzle. The cloud continues to lower (cirro stratus & alto stratus). Rain becomes persistent over a wide band (up to 200 nm ahead of the front) and the cloud lowers further (nimbostratus & stratus), The approach of the cold front is marked by a build up of upper cloud (Cirrus and possibly Cirrostratus) and medium cloud (Nimbostratus) just ahead of the front. Followed by a relatively narrow band of rain. The passage of the front is marked by low cloud and heavy rain. Where the frontal nose collapses, it will also be marked by a line squall. Behind the front the rain continues for up to 50 miles. The cloud then lifts until eventually the skies clear and cumulus clouds begin to form. These may continue to develop vertically to give cumulonimbus clouds with associated rain. Cold sector Cu Cs Ci As St Ns Warm sector Sc Cb 40,000ft 30,000ft SENDING A FIRST SOLO DISCUSS THE FOLLOWING 20,000ft 10,000ft A

Weather changes Passage of an occluded front. The rain band is 100-200 miles wide on a warm occlusion and 50-100 miles wide on a cold occlusion. A cold occlusion may also have embedded cumulonimbus clouds at the front. The passage of an occluded front is generally similar to a warm front with upper cloud gradually being replaced by lower cloud. Cool sector Cu Cs Ci As Cold sector Warm sector Ns St Sc 40,000ft Cb 30,000ft SENDING A FIRST SOLO DISCUSS THE FOLLOWING 20,000ft 10,000ft

THE END Any Questions?