PRESSURE SYSTEMS:The latitude of the British Isles gives us an interesting and unpredictable climate. All year round we can be under the influence of a high or low pressure system that helps shape the weather we receive. High pressure systems like this:
bring different meteorological conditions corresponding to the season they have formed under. If a high pressure system forms over the UK in winter, it brings cold conditions as a result of the absence of cloud cover and therefore there is no rain. If a high pressure system forms during summer, it brings warm conditions in both day and night, clear skies, the possibility of early morning mists that disperse quickly, sea frets or haars in the east coast as winds blow onshore from the North Sea and thunderstorms may occur in areas where the air has a relatively high humidity. High pressure systems tend to stick around for longer in comparison with low pressure systems, and can remain stationary over an area for several days or weeks. Anticyclones are associated with high pressure systems, with air subsiding and warming as it falls. This produces a decrease in its relative humidity which leads to a lack of cloud development and dry conditions. Isobars on synoptic charts (like the one above) are usually far apart, and therefore there is little pressure difference between the centre and the edge of the anticyclone. Winds are weak, and flow gently outwards in a clockwise direction in the northern hemisphere, and anticlockwise in the southern hemisphere.
Low pressure systems are associated with depressions and cyclones (tropical revolving storms such as hurricanes.) They usually form over the Atlantic Ocean and are represented on a synoptic chart by the closeness of the isobars indicating a steep pressure gradient and strong winds. Depressions are formed in the Atlantic when two air masses (in this case, the Polar maritime and the Tropical maritime air masses) meet and a front is formed. This front (known as a polar front) is formed as the warm, moist tropical air that has come from the Atlantic is warmer than the cold air that has come from the Arctic. As the Tropical maritime air is warmer and less dense, it rises over the cold air. In a depression, where the warm air rises over the cold air, a warm front is formed (represented by red semi-circles on a synoptic chart) and in this warm front is the first part of the depression to reach land. After the passing of the warm front, the cold front made up of the denser, polar air squeezes the distance between the warm front and cold front until eventually the cold front catches up with the warm front and an occluded front (either shown by a line with both blue triangles to represent the cold front and red semi-circles or by purple triangles and semi-circles together on the same line) is formed. Low pressure systems tend to bring wet conditions due to the passages of depressions and the cloud coverage means that temperatures are mild.
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