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  What are the causes of low-pressure and high pressure hazards?  

The objective here is to provide students with knowledge of the role of jet streams and Rossby waves in controlling the formation of weather systems.

Linked to this, students need to have knowledge and understanding of the term ‘hazard’ and the causes of low-pressure and high pressure hazards in either tropical or temperate climates.

  "Where has the Ferrel Cell gone - sir?"?  

  What are Rossby waves and jet streams?  
  Source: http://www.s-cool.co.uk/alevel/geography/introduction-to-weather-and-climate/atmospheric-circulation-and-motion.html  
Rossby waves are high altitude, fast moving westerly winds, which often follow an irregular path. The path that they take changes throughout seasons, as shown in the diagram below:


NP = North Pole.

R = Ridge.

T= Trough.

Of greater importance, are jet streams that are found to exist within Rossby waves. They help in the rapid transfer of energy around the globe, as they are very fast, narrow bands of air that can reach speeds of over 200km per hour. Five jet streams exist, with three having significant importance:

Jet stream: Location: Characteristics:
Polar front 40 degrees North and South of the equator. Divides the Ferrel and polar cells. Gives wet or fine weather on Earth's surface, and is strongly associated with anticyclones and depressions.
Subtropical 25-30 degrees North and South of the equator. Divides the Hadley and Ferrel cells.
Easterly equatorial Equatorial regions. A seasonal jet stream
  How were Rossby Waves Discovered?  

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  Made by http://classtools.net/widgets/turningPage_4/xcvrp.htm  

  What controls where they form?  

  Source: Warburton (adapted)  
  It is thought that Rossby waves may be caused by the presence of substantial mountain barriers such as the Rockies, the Andes or the Tibetan plateau. Mountains help to create the wave like pattern. The ridges swing northwards and upwards around the barrier in a ridge and then swing downwards and southwards on the leeward side.  
  What is the purpose of Rossby Waves?  
  Rossby Waves are like rivers of air in the upper troposphere and they gradually meander. The meander loops get bigger and bigger until their wavelength from trough to trough could be as much as 8000 kms. When the Waves are well developed and cover a wide range of latitude they are said to have a low zonal index - which leads to the formation of ridges of blocking, high pressure systems and dry stable conditions. When they are almost straight and cover a narrow zone of latitude they are said to have  a high zonal index - which leads to a succession of low pressure systems and unsettled weather. The waves evolve then they straighten up and then meanders form again in an endless cycle. The wave evolution cycle lasts about 6 weeks. But what is the purpose of this?  


Mouse off = High zonal index
Mouse over = Low zonal index 

Move the mouse over this image to see a contrasting "low zonal index" situation.

  Adapted from Source: http://intranet.st-peters.york.sch.uk/fileadmin/subjects/geography/GeogIntranet/HTMLpages/w_depres.htm  
  The diagram shows that as a wave develops (in the mid-latitudes of the northern hemisphere) cold Polar air is dragged southwards and surrounded by warmer Tropical air. Similarly loops of warmer Tropical air are moving north and being cut-off by cold Polar air. In this way heat transference is occurring - cold air moving south and warming; warmer moving north and cooling. When the loops become very pronounced, they detach the masses of cold, or warm, air that become cyclones (depressions/low pressure) and anticyclones (high pressure) areas that are responsible for day-to-day weather patterns at mid-latitudes.  

  Source: http://www.answers.com/topic/rossby-wave  
  What is the relationship of Rossby Waves to surface high and lows?  

  Source: Warburton  
  Surface high pressure areas and surface low pressures are thought to correlate to particular parts of the wave trough section of the Rossby Waves. The surface conditions in each location is the opposite of what is happening in the Upper Air.  
  How is air movement different in Highs and Lows?  

  Source: Witherick  
  A large proportion of day to day weather conditions (in Temperate areas like the U.K. )are the result of the movement of depressions and anticyclones. In a depression the surface air winds blow inwards and clockwise (convergence) but in the upper air above there is a compensatory outwards movement (divergence). With an anticyclone the surface air moves away clockwise from the centre (divergence) but in the upper air there is convergence. Convergence and divergence provide a vital link between upper air movements in the Rossby waves and surface weather systems.  
  What is the effect of different Rossby wave positions on the UK weather?  
  The jet stream is a current of wind, high in the atmosphere (around 30,000ft), that circles the globe in a series of waves, known as Rossby waves. The track of these waves has a huge impact on the weather conditions around the globe as they are responsible for steering and driving Atlantic low pressure systems.  

  During the autumn and winter months the waves of the jet stream are directed towards the UK, bringing bands of wet and windy weather quite typical of these seasons. However, during the summer, the jet stream usually shifts northwards and steers the depressions away from the UK. This northward shift also allows an area of high pressure, known as the Azores high, to nudge northwards, bringing more summery dry, sunny and warm weather to the UK.  

  Source: http://www.bbc.co.uk/kent/content/articles/2008/09/08/weather_bad_summer_feature.shtml  
  Both in 2009 and last 2008 the jet stream stayed to the south (see above) which continued to steer Atlantic depression after Atlantic depression towards the UK. This meant that the summer was plagued by spells of unseasonably wet and windy conditions which were more apt for the autumn and winter months.  
  Both high zonal flow and low zonal flow can bring wet depressions to the U.K.!  


  Source: http://weatherfaqs.org.uk/node/144  
  Worked answer - How can Rossby waves influence surface weather conditions?  

  Source: Spencer  
  Fun Quiz on Rossby waves  

  What is the function of the mid-latitude waves and identify the main forces that govern the wave pattern? Describe the relationship between upper air circulation and surface anticyclones and depressions. (20)