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BI - ANTICYCLONES
Anticyclones are high pressure weather systems that form over the British Isles frequently. They are characterized by calm winds with widely spaced isobars, sinking air that warms and decreases humidity, causing clear skies. In winter, anticyclones bring cold days and frosts with the potential for pollution inversions, while summer anticyclones result in hot days with hazy sunshine and the risk of thunderstorms from moist air masses. Anticyclones can influence weather conditions for up to 10 days as they move slowly across the region.
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BI - ANTICYCLONES
- 1. Anticyclones ANTICYCLONES -BRITISH ISLES
- 2. Sketch a copyof the anticyclone below. Remember anticyclones form as a result of air piling up aloft. This encourages air to subside / sink down. As the air sinks, it warms, this increases the amount of water the air can hold, and thus keeps the sky relatively cloud free. Leading edge
- 4. Anticyclones, Highs •Anticyclones over the British Isles occur frequently, but are less common than depressions. • Anticyclones give rise to different weather conditions, depending upon the time of year. • Anticyclones move slowly and are stable features. • They can be 3000km wide – much bigger than depressions • High pressure systems tend to be longer lasting than depressions, once ‘blocked’ they can last up 10 days. They deflect depressions away from UK & Western Europe.
- 5. Anticyclones are characterisedby: • High pressure readings in the middle. • Far apart isobars = calm wind. • The air subsides (sinks). It warms adiabatically which leads to a decrease in relative humidity = lack of cloud & dry conditions. • Air moving away from the centre of the high. • Air spins clockwise around from the high. • They normal travel to the British Isles from mainland Europe. • High pressure systems form commonly over the Azores – ‘Azores High’.
- 6. Anticyclones give riseto different conditions, depending on the time of year. Winter • Cold day time temps, from below 0oc to 5oc • Very cold night time temps – frosts below 0oc • Generally clear skies by day & night Summer • Hot day time temp, over 25oc • Warm night time temps • May not fall below 15oc • Generally clear skies by day & night
- 7. Anticyclones give riseto different conditions, depending on the time of year. Winter • Low level cloud may linger and radiation fog may remain in low lying areas • Causes high levels of air pollution in urban areas as pollutants are trapped by temperature inversion Summer • Hazy sunshine in some areas • Early morning mists which disperse quickly • Heavy dew in the mornings • East coast has sea frets or haars (onshore winds) • Thunder storms can occur when RH is high (Spanish Plume)
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