1. Recap……. Name the air masses…..

2. Name the weather…..

4. Major global air masses

5. Depressions

6. What is a Depression? Depressions are a common feature of our climate. They develop in mid latitudes (UK), and their formation is controlled by what is going on in the upper atmosphere. They are significant as they bring about changes as they pass by

7. Depressions are characterised by: Low pressure reading in the middle. Close isobars = strong wind. Air moving toward the centre of the low. Air spins anticlockwise around the low. They normal travel W -> E over the UK, due to the prevailing wind direction. Low pressure systems form commonly over Iceland – ‘Icelandic Low’.

8. To understand depressions fully…. You need to understand the idea of the tri-cell model (areas of high and low pressure) The idea of air masses The idea of Corriolis and how this effect wind You also need to get your head around…..

9. Jet Streams

10. The jet stream consists of ribbons of very strong winds which move weather systems around the globe. Jet streams are found 9- 16 km above the surface of the Earth, just below the tropopause, and can reach speeds of 200 mph. The position of a jet stream varies within the natural fluctuations of the environment. They are caused by the temperature difference between tropical air masses and polar air masses. What happens in one part of the world depends on what is happening elsewhere - the atmosphere is a complete environment with numerous connections.

11. The origins of depressions that affect us are found between Greenland and Iceland – the Icelandic Low. The depressions follow the pattern created by Rossby waves. They form in the ‘troughs’ as air rushes in from the ‘ridges’ (high pressure). Convergence at ground

12. 2009

13. Jet Streams and Rossby Waves Make notes from handout on the above.

15. So, back to depressions……

17. But first, we need to understand how synoptic charts work Fill in the blanks on your copy of the table

18. Describe the current weather in Hull and London

19. The synoptic symbols are used to see current conditions but are less useful in making predictions. For this, we need to use a different map

20. Watch the video and make some notes on using synoptic maps to predict weather. Annotate your map to show the various features and what they mean for the weather

21. A typical depression over the UK

22. Development of Low Pressure

23. FRONTS

24. How do Depressions Develop? Depressions have 3 stages. Each stage brings with it differing conditions Embryo, Mature, Decaying

25. Air Mass Movement When two air masses meet they do not readily mix - they will have differing temperatures and densities. The junction between the air masses is called a Front. A Warm Front occurs when warm air is advancing and being forced to rise over colder air. A Cold Front occurs where the colder air advances to undercut the warmer air mass - forcing it to rise. In both of these situations the rising air becomes colder and condensation occurs producing clouds - easily visible on satellite photographs. Eventually, the cloud may become thick enough to generate precipitation. Fronts extend over many hundred of kilometres and extend upwards at gentle slopes

27. Warm wind Cold wind Warm air from the south meets the colder air from the North Polar Front Early Stage

28. Warm wind Cold wind Warm air rises over the cold air – creating the warm front The cold air undercuts the warm air, creating a steeper cold front. Open Stage

29. Warm wind Cold wind The cold front travels faster, eventually catching up the warm front – forming an occlusion. Occlusion – the cold front over rides the warm front. Occluded Stage

30. Cold wind The occlusion is fully closed – therefore dissolved. Dissolved Stage

31. Embryo

32. Depressions develop from small waves in the polar front (Jet Stream). The polar front is where cold air meets warm air, often cold dry polar continental (Pc) air meeting warm moist tropical maritime (Tm) air. convergence of the air masses results in the warmer air being forced up over the colder air in a spiral motion. The upward movement of the air results in 'less' air at the Earth's surface giving an area of low pressure. The wave develops and the kink becomes more pronounced. The developing depression with its warm front and cold front usually moves in a north-westerly direction under the influence of the upper westerlies.

33. Mature

34. A mature depression is recognised by the cold front starting to catch the warm front up. The pressure continues to fall as more and more air is forced to rise. As the pressure falls and the pressure gradient steepens, the inward blowing winds increase in strength. Due to the Coriolis Force these winds blow anticlockwise.

35. Mature Con’t Pressure in the depression continues to fall as more warm air, now forming a Warm Sector, is forced to rise. As the pressure falls the Pressure Gradient steepens generating inward blowing winds with greater force. In the Northern Hemisphere these winds blow in an anti- clockwise direction as a result of the Coriolis effect. As the warmer air of the warm sector rises along the warm front it cools and eventually reaches its Dew Point at which point condensation occurs producing clouds. Large amounts of Latent Heat are released causing the air temperature to increase further, making the now warmer air rise even more. This continued uplift will create thicker and thicker clouds, eventually resulting in rainfall along the line of the front.

37. Decaying

38. The depression begins to decay when the cold front catches up with the warm front, forming an occluded front, or occlusion. By the time this has happened the warm air has been squeezed right up so it is no longer in contact with the ground. As the uplift of the air is decreased, so too are the amount of condensation, the amount of latent heat produced, and the amount and pattern of precipitation. Often there is only one episode of rain.

39. What Weather???? While each depression may differ slightly, we can associate a certain pattern with the movement of the depression as it passes

41. Weather Using your knowledge and the sheets provided, state the types of weather you would expect as…. Before the warm front, at the warm front, in the warm sector, at the cold front and behind the cold front Think about, temp, pressure, wind, humidity, cloud cover and precipitation

42. PRESSURE

43. WIND SPEED

44. TEMP

45. PRECIPITATION + HUMIDITY

46. QUESTIONS Describe the changes as a depression approaches, moves over, and passes the UK. Use adding and comparing connectives to describe the sequence

47. Homework http://www.metoffice.gov.uk/corporate/library/factsheets.html http://www.answers.com/topic/rossby-wave Review notes on depressions and ensure their formation is understood – above web sites may help.