Urban Microclimates by Aisha JW

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  • Created by: Aishajw
  • Created on: 18-04-20 19:36
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  • Impact of urban areas on the local weather by Aisha Johnson-Witter
    • Winds
      • Lower wind speeds
        • On the whole, urban areas have lower wind speeds than outlying suburbs, on average 5% less than in city centre.
          • This is due to the roughness of the land surface (friction) as a result of buildings
          • In rural areas there is not much variation in wind speed as there is less change in the surrounding areas height.
      • Greater variability
        • In CBD wind speeds are generally more gusty and strong - this is due to building layout and height.
          • Straight streets lined with tall buildings can produce 'Urban Canyons' this funnels wind producing high velocities
            • This is due to the Venturi effect which squeezes airflow as buildings tend to get taller towards the city centre.
              • Turbulence is created by high rise buildings that disrupt the flow of air.
          • Buildings that are designed on stilts and canopies may reduce turbulence
          • Widley spaced buildings act as single blocks, but where buildings are closer together, the airflow around one interferes with the surrounding area.
            • In cities where buildings are densely packed, air can move over the top, leaving the lower urban canopy layer with less turbulence - this is not consistent with buildings of different heights
    • Temperature
      • Urban heat island - urban areas that are warmer than their rural surroundings, at an average of 1-2 degrees per year. for example Melbourne's avergae heat island is 1.13 degrees
        • Peaks in urban heat islands are associated with industry and city centre and troughs with parks and rivers.
      • Heat island effect is caused  by 5 main factors
        • 2) Height and arrangement of buildings
          • Insolation can warm surfaces but depends on how much isnolation is absorbed
          • this depends on how much is reflected off that surface - albedo. Urban and rural generally have similar albedos that are quite low
        • 5) Presence of Pollutants
          • with urban heat islands, pollutants have both a cooling and heating effect.
        • 1) Anthropocentric heat
          • this is the heat released by human activity. heat is injected into the canopy layer and the upper boundary layer.
        • 4) The presence of water
          • surplus of heat can be transmitted as either latent heat or sensible heat. latent evaporates on water  and sensible which is heat energy that can be felt.
    • Pollution
      • 1956 Clean Air act cleared up most urban air
      • urbanmicroclimates are certainly dirtier than rural areas, although this is decreasing
      • London sunshine has shown an increase over the past 100 yeras,
      • photochemical smog has an increasing impact - Los Angeles has become synonymous with photo chemical smog.
        • Slow moving air means that pollutants are able to stagnate within the urban canopy layer. Both PAN and ozone make breating difficult
    • Clouds and Precipitation
      • Urban areas have greater cloud coverage - 5-10% more according to Met Office.
        • This may be due to more condensation nuclei which is actually around 100x more than in rural areas!
      • Untitled
      • precipitation is more common in cities (5-30% more common in rural areas)  Cities are not only wetter but are actually more prone  to thunderstorms
        • Lomdon has between 100 and 110 days of thunder p/year vs Kent which has 60-70 days.
  • 3) Nature of building materials
    • Urban areas heat up more quickly than rural areas

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