Urban Microclimates by Aisha JW
- 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.
- On the whole, urban areas have lower wind speeds than outlying suburbs, on average 5% less than in city centre.
- 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.
- This is due to the Venturi effect which squeezes airflow as buildings tend to get taller towards the city centre.
- 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
- Straight streets lined with tall buildings can produce 'Urban Canyons' this funnels wind producing high velocities
- In CBD wind speeds are generally more gusty and strong - this is due to building layout and height.
- Lower wind speeds
- 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.
- 2) Height and arrangement of buildings
- 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
- 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.
- Urban areas have greater cloud coverage - 5-10% more according to Met Office.
- Winds
- 3) Nature of building materials
- Urban areas heat up more quickly than rural areas
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