CGP Urban Climate

• Urban climate characteristics

  What the weather's like in cities
• Urban Areas are Warmer than the Surrounding Rural Areas
• Urban heat island effect


  The phenomenon of urban areas being warmer than rural areas
• Urban heat islands (UHI)


  Urban areas with higher air temperatures than the surrounding rural areas
• Urban heat island


  • London has a clearly defined UHI
• Highest temperatures


  • Found in industrial areas and in the most densely built up areas, eg, the CBD (Central Business District)
• Temperature sinks


  • Pockets of cool air above parks and bodies of water (eg, rivers or ponds)
• Temperature plateaus


  • Areas within the city with the same land use (eg, industry) generally have the same temperature
• Temperature differences

  • Rapid changes when land use changes (eg from inner city housing to CBD high rise buildings)
• Four Main Causes of the UHI Effect

  • Absorption of heat by urban surfaces
  • Air pollution
  • Heat from human activity
  • Less evapotranspiration
• Absorption of heat by urban surfaces

  Concrete, brick and tarmac surfaces absorb and store heat from the sun during the day, then slowly release it as long wave radiation at night
• Air pollution

  Increases cloud cover and creates a 'pollution dome' that traps outgoing heat radiation and reflects it back to the surface
• Heat from human activity

  Cars, factories, offices, central heating, air conditioning units and people themselves all release heat
• Less evapotranspiration


  When it rains the water's quickly removed by drainage systems, so there's little surface water to evaporate. Also, there isn't much vegetation, so there's little transpiration. Evapotranspiration uses heat energy, so less evapotranspiration means higher temperatures.
• Evapotranspiration


  The transfer of water from the Earth's surface to the air via evaporation and transpiration
• Effects of UHI Vary Seasonally and Diurnally (Between Day and Night)

  • UHI effect is stronger at night
  • UHI effect is stronger in summer
  • UHI effect is stronger when there's an anticyclone
• UHI effect is stronger at night


  Urban daytime temperatures are on average 0.6 °C warmer than surrounding rural areas, but night time temperatures can be 3-4 °C warmer. This is because rural areas cool down at night, but urban areas don't cool as much because urban surfaces continue to release heat that they've absorbed during the day.
• UHI effect is stronger in summer

  Average winter temperatures can be 2 °C warmer but average summer temperatures can be up to 5 °C warmer. This is because there's more solar radiation in summer, so urban areas absorb more heat.
• UHI effect is stronger when there's an anticyclone


  Anticyclones cause clear skies and low winds. If there are no clouds, more solar radiation reaches and heats the ground. Low winds mean warm air isn't blown away.
• Effect of urban structures on wind

  • Average wind speed is generally lower in cities
  • There are areas where wind speed is reduced because they are totally sheltered from wind
  • You get turbulence and eddies where wind is deflected around buildings
  • You get powerful gusts of wind where wind is funnelled down streets - this is known as the canyon effect
• More rain, fog and thunderstorms in urban areas

  The UHI effect means the air in urban areas is warm, and warm air can hold more water. The warm, moist air rises, causing condensation and rainfall. Urban areas also generate lots of dust and pollution, which act as condensation nuclei to encourage cloud formation and fog.
• Less snow and frost in urban areas


  It doesn't snow as often in urban areas, and when it does, the snow melts faster. Urban areas also have fewer days of frost. This is because it's warmer due to the UHI effect.