3. Anticyclones


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3. Anticyclones

  1. 1. Anticyclones have an important and contrasting influence on UK weather. AQA Geog A Textbook page 60 - 61
  2. 2. objectives <ul><li>To describe the weather that accompanies an anticyclone. </li></ul><ul><li>To explain the summer and winter contrasts and reasons for the differences </li></ul>
  3. 3. What is an Anticyclone? <ul><li>Anticyclones affect the British Isles far less often than depressions. </li></ul><ul><li>Anticyclones are areas of high pressure caused by air sinking towards the ground. </li></ul><ul><li>Once Anticyclones develop they tend to remain for several days, bringing dry bright and settled weather to the UK. </li></ul>
  4. 4. Winds in an anticyclone circulate in a clockwise direction in the northern hemisphere
  5. 5. How do they form? <ul><li>Anticyclones form when air is descending. </li></ul><ul><li>As more air descends so the pressure increases. </li></ul><ul><li>This brings very light winds (which blow outwards from the centre in a clockwise motion). </li></ul><ul><li>Descending air is warming air, which means that it picks up moisture from the atmosphere, rather than releasing it through condensation. </li></ul><ul><li>The lack of condensation means that few clouds form and the weather remains fine and dry. (because the air is descending and warming up, there are no clouds and no rain). </li></ul>
  6. 6. What do they look like?
  7. 7. Summer Anticyclones <ul><li>Summer anticyclones bring hot and sunny weather due to there being clear skies (because of the descending air). The days are long and the sun high in the sky, which gives plenty of opportunity for the land to be heated up. These conditions could therefore lead to convectional rainfall occurring. </li></ul>
  8. 8. Winter anticyclones <ul><li>Winter anticyclones bring clear cloudless crisp conditions. However the sun is low in the sky and the days are short. </li></ul><ul><li>At night, heat from the ground is lost due to the absence of a blanket of cloud. These clear skies allow frost to form. </li></ul><ul><li>As the UK is surrounded by the sea the air is often moist. The rapidly cooling ground cools and condenses any moisture in the air above it, forming droplets of ice when the temperature falls below freezing. This is frost. </li></ul><ul><li>Fogs are also caused by clear winter nights. The ground loses heat. This cools the air above it causing moisture to form around dust particles in the layer of air closest to the ground surface. This is fog. </li></ul>
  9. 9. Exam questions AQA A 2004 Q9 Explain the typical weather associated with an anticyclone (6 marks)
  10. 10. Level 1 Basic (1-2 marks) Basic points, may only consider summer or winter and may be a concentration on the weather rather than its explanation . it is hot and dry winds are light little cloud cover frost and fog in winter etc. Level 2 Clear (3-4 marks) Clear statements, may only consider summer or winter e.g. winds are light because isobars are far apart, it is hot as there is no cloud cover, it is dry because no clouds, frost in winter because very cold with clear skies. Level 3 Detailed (5-6 marks) Some reference to both winter and summer and more understanding of atmospheric processes e.g. winds are light because isobars are far apart indicating a low pressure gradient; no cloud cover allows high summer temperatures as there is maximum heating by the sun, in winter the clear skies and long hours of darkness lead to maximum radiation and so temperatures fall below zero making frost likely. Accept explanations of radiation fog, subsiding air. Temperature inversions, blocking anticyclones as appropriate
  11. 11. AQA A 2009 Q9 (3 marks)
  12. 12. <ul><li>1×1 for identifying the presence of an anticyclone / area of high pressure. </li></ul><ul><li>2×1 for describing the weather conditions, e.g. cold overnight, possibly fog or frost, dry, little/no wind, sunny or clear skies, dry. </li></ul>
  13. 13. (4 marks)
  14. 14. <ul><li>Social </li></ul><ul><li>should relate to inconvenience </li></ul><ul><li>plans disrupted </li></ul><ul><li>increased stress </li></ul><ul><li>disappointments / missed reunions </li></ul><ul><li>nowhere to stay </li></ul><ul><li>cramped conditions at airport. </li></ul><ul><li>Economic </li></ul><ul><ul><li>cost of finding somewhere to stay, </li></ul></ul><ul><ul><li>alternative routes via train </li></ul></ul><ul><ul><li>time lost at work due to delays </li></ul></ul><ul><ul><li>Some able to cash in – local hotels/businesses. </li></ul></ul>
  15. 15. <ul><li>Level marking </li></ul><ul><li>Level 1 (Basic) (1-2 marks) </li></ul><ul><li>Simple statements in a random order. </li></ul><ul><li>May rely heavily on text. </li></ul><ul><li>Social and economic not distinguished </li></ul><ul><li>e.g. time wasted due to cancelled flights, nowhere to stay, hotels full. </li></ul><ul><li>Level 2 (Clear) (3-4 marks) </li></ul><ul><li>Will use information in text. </li></ul><ul><li>Will distinguish between social and economic. </li></ul><ul><li>Will develop points </li></ul><ul><li>e.g. Cancelling of so many flights caused huge disruption – many people stuck and unable to get to see families for Christmas. Hotels nearby were full so their profits would have received a boost. </li></ul>
  16. 16. Hot / high temperature / into mid 20s (1) often get a ‘heatwave’(1). Dry (1) Sunny or clear skies (1) calm (1) convectional rain or thunderstorms (1)