Wind

• What causes wind on Earth?

  1. Vertical movements of air caused by uneven heating of the surface and atmosphere of the sun
  2. Air generally moves along a pressure gradient from areas of high pressure to areas of lower pressure, higher gradient = stronger wind
• 3 effects urban structures and layouts have on wind speed, direction and frequency:

  1. The surface area of cities is uneven because of the varying heights of the buildings > buildings exert a powerful frictional drag on air moving over and around them > act as barriers and reduce wind speeds
  2. High-rise building may slow air movement but they channel air into 'canyons' between them > 'venturi effect' > cause powerful gusts
  3. On calm and clear nights when UHI effect is at its greatest, 'convectional processes' draw in strong localised winds from cooler surrounding areas
• Building spacing and its effect on air flow:

  Buildings that are widely spread:

  
• Building spacing and its effect on air flow: 


  Builds close together:

  
• Building spacing and its effect on air flow


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• Complex air flow patterns around tall buildings: Eddies and turbulence:


  Wind can be deflected around sides of buildings, downwards or over the top > this flow of air can create bodies of swirling air called eddies or vortices

  
• Urban canyons create the Venturi Effect: 


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• Summary - Pressure:

  Pressure gradients set up between windward and leeward side of buildings which causes severe eddying of the winds

  • Windward side tends to have high pressure due to air pushing against it
  • Leeward side has low pressure
  • Wind moves around the building to the area of low pressure > causes steep localised pressure gradient > strong winds
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  • When high pressure conditions occur in rural areas > breezes move towards low pressure created in urban area by the rising convectional heat current > air moves from high pressure in rural to low in the city
• Summary: spacing of buildings:

  Spacing of buildings can affect winds

  • Single buildings widely spaced apart act on the wind by themselves
  • Closely spaced buildings work together with each other creating more frictional drag > wind skims over the top and eddy between them
  • Buildings act as wind channels where the wind moves at faster speeds causing pedestrians to be blown over and causing lots of litter > 'Venturi effect' > acceleration of wind occurs as winds are forced to move through narrowing buildings > 'urban canyons' > increase wind speeds as they are funnelled down the canyon