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  Key Idea 4 continued: What are the causes of low-pressure and high pressure hazards?  
 

This lesson: Low pressure system formation and associated hazards of storms, tropical cyclones and tornadoes.

  
     
 

  • In the Tropics Region: In Low Pressure Systems -  Tropical cyclones with Torrential Rain and High Winds.
     

  • Causes – these hazard conditions are generated in exceptionally deep fast moving depressions generated usually towards the end of the hot season (September and October in the Northern Hemisphere) over oceans off the east margins of continents in the tropics and sub tropics.
     

  • Triggering secondary hazards of flooding, tidal waves and sea incursions, landslides, mudflows and wind borne debris.

  
  Class note                                    PowerPoint                Spider  diagram  
  What are Tropical Revolving Storms?  
 

 
  They are a major hazard in the tropics. Every year about 80 occurrences claim on average about 20,000 lives and cause immense economic damage to property, crops and shipping. A Tropical Revolving Storm is a large rotating storm around a centre of very low pressure. Most systems have a diameter of about 650 km, less than half that of a a mid-latitude depression. Wind speeds often exceed 120kph and can reach 200kph. The huge amount of heat required to  create and maintain a tropical cyclone is reflected by the height of the clouds near its centre, often up to 12km above the surface.  
 

 

  
  Where do they develop?  
 

 
     
 

 
  Source: Global Future Options: Heinemann  
  Make three statements about the areas of ORIGIN of Tropical Revolving Storms.

Make three statements about the TRACKS of Tropical Revolving Storms.

  
     
  Research: find out why hurricanes (aka tropical cyclones, tropical revolving storms) do not form within 5* of the equator.  
     
  How do Tropical Revolving Storms develop?  
  The development of a tropical cyclone is dependent on combinations of particular mechanisms. Click on Pic:  
 

 
  Factors Affecting the development of tropical cyclones  
 

 
  Source: Nagle  
     
  Now how do hurricanes develop?...  
  Animation of how a hurricane develops:  
 

 
  Source: http://news.bbc.co.uk/1/hi/sci/tech/4588149.stm hurricane  
     
  Hurricanes: animated book  
   

Click here for full screen version

  
  Made with http://classtools.net/widgets/turningPage_5/GmrJx.htm Source: Waugh  
     
  The Walker East-West Cell  
 

 
  Source: Warburton  
 
The African easterly jet, is a region of the lower troposphere over West Africa where the seasonal mean wind speed is maximum and easterly. The jet develops because heating of the West African land mass during the Northern Hemisphere summer creates a surface temperature and moisture gradient from the Gulf of Guinea and the Sahara, and the atmosphere responds by generating vertical wind shear to maintain thermal wind balance. During the mature phase of the West African Monsoon (August to September) maximum mean wind speeds in the jet of approximately 13 m/s are located around 4°N—5°N at a height of 4 km (or 650 mb).
  
     
 

 
 
African easterly waves, or Tropical waves in the Atlantic basin develop from disturbances which drift off the continent of Africa onto the Atlantic ocean. These are generated or enhanced by the African Easterly Jet. The clockwise circulation of the large transoceanic high-pressure cell or anticyclone centred near the Azores islands impels easterly waves away from the coastal areas of Africa towards North America. The easterly waves are one of the main conditions necessary for the development of hurricanes. It is an area of Low Pressure which is basically an extension of the ITCZ which lies roughly north to south in the Trade wind belt. These troughs of low pressure are most common in the late summer months. The low pressure of the Easterly Wave often breaks down the upper air flow, allowing moist air to rise into the upper atmosphere.

Approximately 60% of Atlantic tropical cyclones originate from tropical waves, while approximately 85% of intense Atlantic hurricanes (Category 3 and greater) develop from tropical waves. About 100 of the wave depressions occur each year and only about 10 develop into major hurricanes.

  
  Source: http://en.wikipedia.org/wiki/Tropical_wave  
     
  Features of a Hurricane.  
 

 
  Using information from diagrams 1), ii) and iii) above and the two images below describe the main features of a hurricane,  
 

 
  Satellite image of Hurricane Floyd 1999  
 

 
  Source: http://www.physicalgeography.net/fundamentals/7u.html  
     
  Weather associated with the passage of a hurricane  
 

 
  Source: Waugh  
  Describe the weather changes that occur as a hurricane passes over.  
     
  What are the impacts of hurricanes?  
 

 
     
  Hurricane case study: Hurricane Mitch 1998  
 

 
  Source : Waugh  
 

 

Click here for full screen version

  
  Made with http://classtools.net/widgets/turningPage_3/0XISC.htm Source: Waugh  
     
     
  Tornadoes  
 

 
  Source: http://news.bbc.co.uk/1/hi/sci/tech/7533941.stm  
     
  Features of a Tornado  
 

 
  Source: Nagle  
     
  How a tornado develops:  
   

Click here for full screen version

  
  Made with: http://classtools.net/widgets/turningPage_6/wU6A0.htm Source: Nagle  
     
  Case Study:  
 

 
     
  Essays:  
 

With reference to your chosen climatic region, explain the formation of low pressure systems and describe their associated weather patterns. (20) Legacy Jan 2009

 

What are the causes of low pressure hazards, and what factors influence the impact that they have, in either tropical or temperate regions. (GeogOnline)

 

Examine the factors that influence the level of hazard and element of risk associated with low pressure systems in Tropical regions (20) Adapted from Legacy Jun 2009