Urban Microclimates

HideShow resource information
View mindmap
  • Urban Microclimates
    • Winds
      • Lower speeds
        • On average the wind speed in a city centre is 5% less than in the suburbs
        • This is because of higher surface friction such as buildings
      • Greater Variablilty
        • Straight streets can produce 'urban canyons' so the wind funnels down them with high velocity.
          • Venturi Effect
        • Turbulence of air is caused by high-rise buildings disrupting the flow of air
        • Building shape and spacing affect the behaviour of wind
          • Widely spaced buildings act as single blocks whereas buildings that are closer together interferes with the air flowaround the next building
          • Densely packed buildings allow air to move over the top which means the lower urban canopy layer has less turbulence
        • Building design on stilts and canopies may reduce turbulence and gustiness
          • Turbulence of air is caused by high-rise buildings disrupting the flow of air
      • Large Scale Convection
        • Heat Island affect leads to a lowered air pressure
          • Air is drawn in from surrounding rural areas
    • Temperature
      • The Urban Heat Isalnd
        • This is when Urban Areas are on average 1-2 degrees warmer than their surrounding rural areas.
        • Heat island effect is caused by five main factors
          • 1) Anthropogenicheat
            • Heat released from human activity e.g. vehicles, central heating and industrial activities
            • Injects heat into the canopy layer and the upper boundary layer
          • 2) Height and arrangement of buildings
            • Insolation can warm surfaces  but depends on how much insolation is absorbed
              • This depends on a surfaces albedo
                • How reflective a surface is
                • Example: Asphalt has a lower albedo than grass
                • High-rise buildings reduce the albedo, so oveall urban areas have 15% lower albedo than rural areas
            • Once a surface has absobed sunlight it will warm up and give off heat
              • In urban areas the heat is intercepts by other buildings so less heat is lost to the atmosphere
          • Nature of the building material
            • Urban Areas heat up more quickly than rural areas
              • The specific heat capacity for concrete is one-third of wet mud- so less energy input is needed
          • 4) The presence of water
            • If there is little water, then less heat is used for evapotranspiration and the heat will be used to heat up air temperature
          • 5) Presence of Pollutants
            • Photochemical smogs
            • Suspended particulate metter and smoke can reduce the amount of sunshine that reaches the city surface by up to 30%
            • The pollutant dome can absorb heat and prevent it from escaping
              • greenhouse gases such as CO2 and Ozone warms up from insolation but warms up even more by re-radiating the heat back onto the urban surface
      • Sensible heat  goes into heating the atmosphere and can be felt whereas latent heat cannot be felt and goes into evapotating water
    • Clouds and precipitation
      • Urban areas have a greater cloud cover
        • This may be due to a greater concentration of condensation nuclei
          • Cities have high heat which increates uplif of air by convection, encouraging cloud formation
      • Precipitationis more common in urban areas by 5-30%
        • Cities are designed to remove more surface water so humidity during the day is lower than rural areas
          • At night the humidity in rural areas drops rapidly so in urban areas the humidity is higher
      • Storms are more likely in cities
    • Pollution
      • Urban areas have more pollution than rural areas do
      • Poluution in urban areas is being reduced
        • Soot was emitted from coal-fired sources
          • The amount of sunshhine recieved increasef
      • Worse in the winter as coal is used for heating
      • Clean Air Act - 1956
      • Pan and Ozone causes breathing difficulting and eye irritations
      • Photochemical Smogs


No comments have yet been made

Similar Geography resources:

See all Geography resources »See all Rural and urban challenges and regeneration resources »