URBAN MICROCLIMATES
Human activity can have a big impact on the climate of local areas. One such way is through the creation of large urban areas, with a high density of human created structures. With the amount of buildings, the colours and materials used, factories and the presence of large numbers of people, the climate - the long term weather behaviour of an area, including factors such as temperature, precipitation, air quality, pressure and winds - can be affected.
Temperature: Urban heat islands
Due to human activity, the temperature in an urban microclimate is higher than the surrounding areas, and what it should usually be. Urban areas are said to be urban heat islands. This is especially noticeable under calm conditions, where the city centre will be several degrees warmer than the countryside. There are several reasons why this happens:
- building materials are non-reflective so absorb more heat than natural surfaces and can store heat
- road surfaces such as concrete and tarmac have a high thermal capacity, and their dark colour means they absorb a lot of heat during the day, which is released slowly at night, increasing night temperatures
- presence of factories and high car usage gives off a lot of heat
- they also give off pollution and smog, and a pollution dome forms. This allows short-wave insolation to enter, but traps longer wave, outgoing terrestrial radiation, which increases the amount of heat contained
Due to these reasons, the temperature in winter in urban areas are on average 1-2 degrees Celsius higher than rural areas. The average summer temperatures are on average 5 degrees Celsius higher than rural areas. This can be seen in the diagram below which depicts London's heat island.
Due to human activity, the temperature in an urban microclimate is higher than the surrounding areas, and what it should usually be. Urban areas are said to be urban heat islands. This is especially noticeable under calm conditions, where the city centre will be several degrees warmer than the countryside. There are several reasons why this happens:
- building materials are non-reflective so absorb more heat than natural surfaces and can store heat
- road surfaces such as concrete and tarmac have a high thermal capacity, and their dark colour means they absorb a lot of heat during the day, which is released slowly at night, increasing night temperatures
- presence of factories and high car usage gives off a lot of heat
- they also give off pollution and smog, and a pollution dome forms. This allows short-wave insolation to enter, but traps longer wave, outgoing terrestrial radiation, which increases the amount of heat contained
Due to these reasons, the temperature in winter in urban areas are on average 1-2 degrees Celsius higher than rural areas. The average summer temperatures are on average 5 degrees Celsius higher than rural areas. This can be seen in the diagram below which depicts London's heat island.
Atmospheric Pressure and Winds
Urban heat islands also have an effect on atmospheric pressure and winds. As warm air rises over an urban area it draws air in from the surrounding area and creates an area of localised low pressure. A pressure gradient develops between higher pressure colder rural areas and warmer urban areas, and winds develop to equalise this gradient. These winds converge on urban areas, and may bring pollution from outer areas into the city centre.
The winds are also affected by the urban surroundings - the layout and the structure of the buildings. Tall buildings in particular, affect the way air moves across the city and reduce wind velocity. Tall buildings also provide frictional drag on air movement, which creates turbulence, which changes the direction and speeds of the wind. Pressure gradients are also established between the windward and leeward sides of buildings. The windward side, the side facing the direction of the wind, tends to have high pressure due to the air pushing against it, and the leeward side, the side away from the wind has an area of low pressure. This causes a steep localised pressure gradient.
The spacing of buildings also impacts the winds. Closely spaced buildings work together to create more friction drag which causes eddying in between them. Buildings can also act as wind channels, so the wind is stronger and faster. For example, Chicago is nick-named the windy city due to its grid system of buildings creating wind tunnels.
Urban heat islands also have an effect on atmospheric pressure and winds. As warm air rises over an urban area it draws air in from the surrounding area and creates an area of localised low pressure. A pressure gradient develops between higher pressure colder rural areas and warmer urban areas, and winds develop to equalise this gradient. These winds converge on urban areas, and may bring pollution from outer areas into the city centre.
The winds are also affected by the urban surroundings - the layout and the structure of the buildings. Tall buildings in particular, affect the way air moves across the city and reduce wind velocity. Tall buildings also provide frictional drag on air movement, which creates turbulence, which changes the direction and speeds of the wind. Pressure gradients are also established between the windward and leeward sides of buildings. The windward side, the side facing the direction of the wind, tends to have high pressure due to the air pushing against it, and the leeward side, the side away from the wind has an area of low pressure. This causes a steep localised pressure gradient.
The spacing of buildings also impacts the winds. Closely spaced buildings work together to create more friction drag which causes eddying in between them. Buildings can also act as wind channels, so the wind is stronger and faster. For example, Chicago is nick-named the windy city due to its grid system of buildings creating wind tunnels.
Precipitation
Precipitation is also affected in an urban microclimate. There is more precipitation in urban areas than rural areas, about 10-15%. They do have a larger amount of dry days, yet have more rainfall when it does rain. This is because concrete surfaces lead to convection currents and more cloud formation. Because of the higher temperatures in urban heat islands, there is a 25% increased likelihood of thunderstorms, and a higher chance of snow being turned into sleet, decreasing the number of snow days by 15%. The intensity, frequency and length of fogs are much greater in urban areas, especially under anticyclone conditions
Precipitation is also affected in an urban microclimate. There is more precipitation in urban areas than rural areas, about 10-15%. They do have a larger amount of dry days, yet have more rainfall when it does rain. This is because concrete surfaces lead to convection currents and more cloud formation. Because of the higher temperatures in urban heat islands, there is a 25% increased likelihood of thunderstorms, and a higher chance of snow being turned into sleet, decreasing the number of snow days by 15%. The intensity, frequency and length of fogs are much greater in urban areas, especially under anticyclone conditions
Air quality and cloud cover
Winds from rural areas can bring in pollutants, and the higher temperatures means there is more condensation and therefore up to 10% more frequent cloud cover. There is also more pollution in urban areas, with more dust from car fuel and industry. Cloud cover may also result from photochemical smog and smoke. Cloud cover will then cause more heat to be trapped, raising the temperature further
Winds from rural areas can bring in pollutants, and the higher temperatures means there is more condensation and therefore up to 10% more frequent cloud cover. There is also more pollution in urban areas, with more dust from car fuel and industry. Cloud cover may also result from photochemical smog and smoke. Cloud cover will then cause more heat to be trapped, raising the temperature further
Humidity
Humidity is lower in urban areas as higher temperatures means the air can hold greater amounts of water vapour. However, the lack of vegetation and lots of concrete surfaces means there is less evapotranspiration.
Humidity is lower in urban areas as higher temperatures means the air can hold greater amounts of water vapour. However, the lack of vegetation and lots of concrete surfaces means there is less evapotranspiration.
Sunlight
Temperatures are higher in urban areas, but the amount of sunlight they receive is actually lower, because high-rise buildings block out the sunlight and higher amounts of dust reflect and absorb sunlight.
Temperatures are higher in urban areas, but the amount of sunlight they receive is actually lower, because high-rise buildings block out the sunlight and higher amounts of dust reflect and absorb sunlight.