Lesson 1: Depressions and Anticyclones


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Lesson 1: Depressions and Anticyclones

  1. 1. Depressions and Anticyclones Mr. T. Tonna
  2. 2. Index • Key Words • What are Depressions & Anticyclones? • What are weather Charts? • Isobars • Fronts • Warm Sector • Different Weather Conditions • Forecasting • Class Task • Summary Mr. T. Tonna
  3. 3. Key Words Depression Anticyclone Warm Front Cold Front Occluded Front Warm Sector Mr. T. Tonna
  4. 4. What are Depressions • ‘Normal’ air pressure is 1016mb. • Depressions are low pressure systems that form in the mid- latitude region (30 -60 N/S) when warm air meets cold air. • When this occurs the warm air is forced above the colder air. It then cools and clouds form which produce rain. • Winds in depressions move in an anticlockwise direction in the northern hemisphere and clockwise in the southern hemisphere. • Characterised by fronts. Mr. T. Tonna
  5. 5. Anticyclones • Anticyclones are the opposite of depressions. • They are high pressure systems • Less common than depressions. • They usually bring nice weather. • Air descends, warms and acquires moisture. • Clear skies and little chance of rain. • If ground is cold low fog can occur. Mr. T. Tonna
  6. 6. Isobars • Isobars are lines of equal atmospheric pressure drawn on a meteorological map. Each line passes through a pressure of a given value, provided certain rules are followed. The rules for drawing isobars are- – Isobar lines may never cross or touch. – Isobar lines may only pass through pressures of 1000 + or - 4. In other words, allowable lines are 992, 996, 1000, 1004, 1008, and so on. – The atmospheric pressure is given in millibars (mb – Pressure lines are usually corrected or sea level so any differences in pressure due to altitude are ignored. Mr. T. Tonna
  7. 7. Fronts Warm Front Cold Front Occluded Front The Beginning of the depression The End of the Depression When the warm and cold fronts join. A Warm Front is labelled with a Red Semi-Circle. A Cold Front is labelled with a Blue Triangle. Occluded fronts are labelled with a mix of the two fronts. Mr. T. Tonna
  8. 8. Warm Sector • The warm sector is the area between the cold and warm front. It is usually associated with calm weather. Mr. T. Tonna
  9. 9. Weather Conditions in Depressions Mr. T. Tonna Precipitation/ Clouds Temperature Wind (Speed/ Strength) Visibility Before Depression No Rain, Cirrus / Cirrostratus Clouds Approx. 6 Degrees Calm, North Easterly Wind Good Approaching Warm Front Light Drizzle, Altostratus Clouds Approx. 6 Degrees Strong, Southerly Winds Good to Fair Arrival of Warm Front Continuous rain, Nimbostratus Clouds Slight Fall in Temperature South-Westerly Force 3-4 Worsens Warm Sector Little Rain, Patchy Stratus Clouds Rises to around 12 Degrees Westerly, Force 3 Limited Passing Cold Front Heavy Showers, Cumulonimbus Clouds. Falls Further North-Westerly, Force 8 Limited Post- Depression Showers, Cumulous Clouds clearing Low 3-4 Degrees. North-Westerly Force 6 Good
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  11. 11. Forecasting Weather Patterns • The Following Weather Charts will show areas experiencing a Mid-Latitude Depression. • Our Task is to analyse the chart and determine which weather characteristics a marked area is experiencing. • As well as this we shall discuss the future weather that area will be experiencing according to the mentioned weather characteristics of depressions. Mr. T. Tonna
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  16. 16. Class Task – Read Pages 204-206 (A New Wider World- Waugh, D; Bushell, T) – Answer the following: • A Low pressure system is called a ________? • True or False: Anticyclones have pressures below 1016mb? • Are Depressions are associate with nice weather? • Warm air moves _____ cooler air during a depression. • London is currently in the warm sector of a depression what kind of weather would you expect there to be? Mr. T. Tonna
  17. 17. Summary • Depressions and Anticyclones are caused by pressure change. • Depressions are more common than Anticyclones in the Mid-Latitude regions. • Depressions bring complex whether phenomena with alternating bursts of showers and drizzles. • Using weather maps we are able to locate depressions and predict the weather of a certain area/locality. Mr. T. Tonna