2. Depressions


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2. Depressions

  1. 1. Depressions have an important influence on UK weather . AQA Geog A Textbook page 57 - 59
  2. 2. Objectives: <ul><li>To describe the sequence of weather associated with the passage of a depression and be able to explain the reasons for it. </li></ul>
  3. 3. A depression…. <ul><li>Is an area of low pressure </li></ul><ul><li>An isolated storm that forms at the boundary of cold polar air moving south and warm tropical air moving north this leads to mild changeable weather. </li></ul><ul><li>Air is rising and this leads to the formation of cloud and rain </li></ul><ul><li>Winds in a depression in the northern hemisphere circulate in an anticlockwise direction </li></ul><ul><li>Brings unsettled weather </li></ul>
  4. 6. Centre of the depression Warm front Warm sector Cold front
  5. 7. How a depression forms…. <ul><li>Depressions form where warm air from the tropics meets and is forced to rise above cold air from the poles. </li></ul><ul><li>The rising air creates low pressure. Air carried by surface winds is drawn into the centre of the depression to replace the rising air. </li></ul><ul><li>The line that separates the two air masses is called a front </li></ul><ul><li>The leading front is the warm front and marks the front of warmer air </li></ul><ul><li>The cold front marks the front of colder air </li></ul><ul><li>At the fronts air is forced to rise – often forming bands of cloud and rain </li></ul><ul><li>Over time the cold front catches up with the warm front to form a single boundary called an occluded front </li></ul>
  6. 11. Weather associated with depressions <ul><li>Ahead of the warm front clouds gather </li></ul><ul><li>A period of prolonged period of steady rain sets in </li></ul><ul><li>Behind the warm front the temperature increases in the warm sector. It remains cloudy with patchy rain and drizzle </li></ul><ul><li>Next comes the cold front. This brings a short period of heavy rain with gusty winds. </li></ul><ul><li>Behind the cold front the temperature falls </li></ul><ul><li>The rain stops </li></ul>
  7. 12. <ul><li>AQA geog pg 59 </li></ul>
  8. 13. Plenary <ul><li>Short period of heavy rain and gusty winds </li></ul><ul><li>Cloud gradually thickens and sun becomes hazy </li></ul><ul><li>Cold and showery </li></ul><ul><li>Warm and cloudy with patch rain </li></ul><ul><li>Prolonged period of steady rain </li></ul>
  9. 14. AQA A 2004 P1 H
  10. 15. <ul><li>2x1 for correct labels (2 marks) </li></ul><ul><li>(ii) Statements with reference to pattern. There is a high pressure area to the south east (over 1020 mbs). </li></ul><ul><li>Pressure decreases to the north west where </li></ul><ul><li>there is a centre of low pressure at under 960 mbs etc. </li></ul><ul><li>Credit reference to pressure gradients/values as developed points </li></ul>
  11. 16. Exam Questions AQA A 2006 Q9A Possible labels – centre of low pressure dense patch of cloud thick cloud shower clouds Scattered clouds Warm sector anticlockwise movement clear sky swirl (of cloud).
  12. 17. <ul><li>AQA A 2006 Q9A </li></ul><ul><li>(ii) Describe and explain the weather associated with the passage of a depression across the British Isles. </li></ul><ul><li>(6 marks) </li></ul>
  13. 18. <ul><li>Level 1 (1-2 marks) </li></ul><ul><li>Simple statements. </li></ul><ul><li>Little reference to sequence/likely to describe or explain </li></ul><ul><li>e.g. it rains when the fronts pass, it gets warmer in the middle. </li></ul><ul><li>Level 2 (3-4 marks) </li></ul><ul><li>Clear statements. </li></ul><ul><li>More idea of sequence and weather, clear statements but likely to be </li></ul><ul><li>incomplete, may be unbalanced, e.g. as the warm front passes there is a long period of steady rain, followed by the cold front with heavy rain. </li></ul><ul><li>Level 3 (5-6 marks) </li></ul><ul><li>Detailed statements. </li></ul><ul><li>Detailed coverage of both description and explanation, e.g. begins with approach of cirrus cloud, cloud base gradually lowers and thickens generating a long period of light rain ahead of the warm front. As the warm front passes temperatures rise and the sky clears due to the presence of the warm sector. As the cold front approaches the cold air undercuts the warm air causing rapid uplift, towering cumulus clouds and a short period of heavy rain due to the steepness of the front ….. 6 marks </li></ul>
  14. 19. <ul><li>AQA A 2008 </li></ul>Q 9 (a) (i) On Figure 17 , mark with an arrow and label: the centre of the low pressure the cold front the pressure along isobar X . 3 × 1 for identifying the centre of the low (L) . within 984, the cold front (WF) and the value of the isobar . 1008. x
  15. 20. <ul><li>Q9 (a) (ii) Explain the changes in cloud cover and rainfall as the warm front passes overhead . (4 marks) </li></ul>Should refer to increasing amount of cloud; layer / stratus cloud type; due to warm air rising over cold air. This cools as it rises. Condensation occurs at increasingly low levels creating layers of cloud. Prolonged rain; light rain; due to low level cloud formation Air can no longer hold increasing moisture and rainfall results. 4 marks Level 1 (1-2 marks) Basic statements in a random order will emphasise description e.g. it will get cloudy and rain. Level 2 (3-4 marks) Clear, more precise statements recognising passage of warm front; explanation will dominate e.g. stratus cloud will increase due to warm air rising over cold air at the warm front. As this continues, light rain will result as the air can hold no further moisture and there will be a fairly long period of light rain