Urban Microclimates
- Created by: kerry
- Created on: 17-04-13 14:32
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- 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
- Straight streets can produce 'urban canyons' so the wind funnels down them with high velocity.
- Large Scale Convection
- Heat Island affect leads to a lowered air pressure
- Air is drawn in from surrounding rural areas
- Heat Island affect leads to a lowered air pressure
- Lower speeds
- 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
- This depends on a surfaces albedo
- 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
- Insolation can warm surfaces but depends on how much insolation is absorbed
- 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
- Urban Areas heat up more quickly than rural areas
- 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
- 1) Anthropogenicheat
- Sensible heat goes into heating the atmosphere and can be felt whereas latent heat cannot be felt and goes into evapotating water
- The Urban Heat Isalnd
- 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
- This may be due to a greater concentration of condensation nuclei
- 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
- Cities are designed to remove more surface water so humidity during the day is lower than rural areas
- Storms are more likely in cities
- Urban areas have a greater cloud cover
- 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
- Soot was emitted from coal-fired sources
- 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
- Winds
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