How Urban Areas Effect Climate

Revision Cards for Urban Heat Islands/ How Urban Areas Affect Climate. 

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Urban Climatic Zone


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In Urban Areas:

  • 5-10% less sunshine.
  • Annual mean temperature of 0.5-1c higher.
  • Winter maximum temp 1-2c higher.
  • 2-3weeks fewer of frosts.
  • Humidity is 2% lower in winter.
  • Humidity is 8-10% lower in summer.
  • 100% more fog & smog.
  • 5-10% more precipitation.
  • 14% less sonw.
  • 5-10% more cloud cover.

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Morphology of Urban Heat Island

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Surface Reflectivity

Surface reflectivity & absorption mean urban temps. are higher by 0.5c- 1c. due to artificial surface, dark surfaces as oppose to natural surface e.g trees that offer shade, evapotransipiration  and absorb CO2 from the air.


  • Trees 0.15
  • Road Surfaces 0.1
  • White painted buildings- 0.7
  • Brick 0.3

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Rain Shadow

  • There is 5-10% more colud cover & precipitation in urban areas.
  • The fact the is rising over urban areas causes more rain over then urban area.
  • This causes a 'rain shadow' behind the urban area where the air is dry & falling.
  • An example of this is Death Valley in California which has become dryer since urbanisation in California.

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Airflow modified by a building

  • Due to high topography of buildings causing friction of wind.
  • This is helped by the grid patten seen in many USA e.g Manhattan and worsened by cities that are randomly layed out such as London.
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Smog: Photochemical & Particulayt

PHOTOCHEMICAL SMOG: Due to a chemical reaction between NOx gases, volatile organic compounds and sunlight. Increased due to modern industrialisation and is common in cities e.g. Mexico City.

PARTICULATE SMOG: Due to smoke, soot and SO2 being trapped close to ground level. This is what occured during the 'Great London Smog' in 1952.

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The Great London Smog 1952

  • Lasted 5 days.
  • Killed 4,000 Londoners.
  • Caused by coal being burnt & SO2 being released.
  • UKs worst air pollution disaster.
  • Led to the 1956 Clean Air Act.

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How Smog is Formed, Los Angeles, USA

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Clean Air Act 1956

  • Response to th Great London smog of 1952.
  • Helped make air 'cleaner' because industry polluted it so much.
  • Reduced smoke from burning coal due to targets being set by governments.
  • Has been constantly updated througout the last decades.

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How London is Reducing the Urban Heat Island Effec

  • COOL ROOVES- Dark rooves are generally 50-60c, cool rooves are highly solar reflective and reduce heat transfer.
  • GREEN ROOVES- Grow plants on a waterproof membrane. 20-40c cooler than dark rooves. reduce heat transfer, rain water & insulate homes.
  • PLANT TREES & VEGETATION- Cost effective, naturally modify heat, provide shade & absorb CO2, rainwater & suns solar heat.
  • COOL PAVEMENTS- Pavements & parking areas with high solar reflectivity and good permibility. 

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  • Particles more than 10 microns in size.
  • Legistration in developing countries is reducing PM10s.
  • Dust control on roads, construction & landfills.
  • Cleaner burning of diesel & gasoline (catalytic converters.)
  • Landscaping, barriers & fencing prevent windblown dust.
  • Control on industrial facilities.

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Philadelphia- Heatwave Preparedness Plan

1. Use mass media to encourage friends & neighbours to visit elderly daily. 2. Activate telephone hotline to provide councilling. 3.Organise health visits to those that require attention. 4. Inform care homes of high risk heat situation. 5. Increase fire & health department emergency staffing. 6. Implement daytime out reach services to the homeless.

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Global Dimming

  • Reduction in inradiance of Earths surface. It's thought to have been due to increase in particulates e.g Sulfates & aerosols.
  • Worldwide 4% reduction over 3 decades (1960-90.)
  • Dimming causes cooling effct and may mask global dimming.
  • Deliberate manipulation of the diming effect is considered a geoengineering technique to reduce global warming impact.
  • Dimming has also interfered with hydrological cycle by reducing rainfall in some areas due to evaporation. 

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