UHI

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  • Urban Climate
    • URBAN AREAS ARE WARMER THAN SURROUNDING AREAS
      • called urban heat island effect
      • urban areas with higher air temp than surrounding rural area are called urban heat islands
        • e.g. London
      • highest temps. found in industrial areas and densely built up areas (CBD)
      • there are pockets of cool air above parks and bodies of water, these are called temperature sinks
      • areas within city with same land use(e.g. Industry) generally have same temp.- called temperature plateaus
      • Temp can change rapidly when land use changes--inner city to inner city housing- known as temperature cliffs
    • 4 MAIN CAUSES OF UHI EFFECT
      • Absorption of heat by urban surfaces
        • concrete, brick and tarmac surfaces absorb and store heat from sun during day
          • they slowly release the heat as long wave radiation at night
      • Air Pollution
        • from cars and factories increase cloud cover over city
          • creates a pollution dome- a layer of pollution over city
            • these trap heat radiation and reflect it back to surface
      • Heat from Human Activity
        • cars, factories, air conditioning and people themselves release heat
      • Less Evapotranspiration
        • when it rains waters quickly removed by drainage systems so there is little surface water to evaporate
          • less vegetation so there's little transpiration
            • evapotranspiration uses heat energy so less evapotranspiration= increase temps.
    • EFFECT VARIES SEASONALLY AND DIURNALLY (DAY AND NIGHT)
      • effect is stronger at night
        • nighttime can be 3-4oC wamrer
          • due to rural areas cooling down at night where as urban areas don't cools as much as they release heat which has been absorbed by tarmac etc
      • effect is stronger in Summer
        • upto 5oC warmer
          • due to more solar radiation in summer so urban areas absorb more heat
      • effect is stronger when there's an Anticyclone
        • anticyclones cause clear skies and low winds= more solar radiation reaches heating round.
        • low winds= warm air isn't blown away
    • WINDS ARE AFFECTED BY BUILDINGS
      • Average wind speed is usually lower in cities than rural areas. Due to tall buildngs creating friction that slows down moving air
        • there are areas where wind speed is zero because some areas are totally sheltered from wind by buildings
      • you get turbulence around buildings
        • this happens when wind hits the face of a building
          • some is deflected down, some around sides and some over top
            • when these winds hit other buildings or the ground they cause vortices (bodies of swirling air)
      • powerful gusts of wind when channeled down streets--known as Canyon Effect
        • CHICAGO- WINDY CITY
    • MORE RAIN, FOG AND THUNDERSTORMS
      • rain more often in urban area than countryside
        • rain is also more intense and more thunderstorms
          • 2 MAiN REASONS FOR THIS
            • UHI effect means air is warm and warm air holds more water. this warm moist air rises-called convectional uplift As it rises it cools water vapour condenses and it rains- CONVECTIONAL RAINFALL
            • Urban areas create large amounts of dust and pollution. Particles of dust and pollution acts as condensation nuclei(trigger water to condense around them) This encourages clouds to form rather than allowing warm moist air to disperse

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