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Urban ecosystems

The urban microclimate

Urban heat islands

Paris skyline

Tall buildings in Paris

A dome of warm air frequently builds up over city centres forming an urban heat island. As a result, temperatures in the city can be several degrees celsius higher than temperatures in the surrounding countryside. There are two main reasons.

The following factors can also contribute.

Precipitation and humidity

As well as being warmer urban ecosystems often have more rain. We have already seen how atmospheric pollution can lead to more cloud cover.

The warmer temperatures can also lead to the formation of convection currents and strong thermals. These can help both to form and strengthen thunderstorms leading to higher rainfall both in the urban area and downwind of it.

For example, between 1951 and 1960 thunder was heard on 110 days in central London but only on 60 days on the Kent coast

Even though there is more rain, air humidity may be lower as fewer bodies of open water mean there is less water evaporating into the air.

Atmospheric pollution

Photochemical haze over London

A distant view of London from the North Downs
showing a photochemical haze

The urban atmosphere often has high pollution levels. Major sources are the burning of fossil fuels, car exhaust fumes and the use of CFCs (chlorofluorocarbons) e.g. as refrigerants and in aerosol sprays.

Photochemical smogs can sometimes be seen over large cities. These form when oxides of nitrogen and unburnt hydrocarbons from vehicle exhaust react with sunlight and ozone in the atmosphere. Asthma and other breathing problems may be exacerbated.

Pollution can cause many other problems including

All such effects can directly or indirectly affect wildlife and their habitats

Light

Cloud cover and pollution smogs will reduce the amount of solar radiation entering the urban ecosystem whilst shade from tall buildings may significantly reduce the amount of light reaching some areas. It is at night however that the biggest difference between urban and rural areas is seen. Night lights around the world shows how world's cities are lit up at night, with their glow extending over the countryside around them.

Wind

Tall buildings provide shelter from the fast moving winds in the upper atmosphere.

At the same time however buildings can disturb the pattern of airflow. Fast turbulent winds can be funnelled between buildings creating eddies and vortices which swirl up dust and litter making walking between buildings difficult and sometimes dangerous.

Microclimate and wildlife

The urban microclimate has a number of distinctive features but it is difficult to generalise about what affect these might have on wildlife because of the many other factors that influence survival in an urban ecosystem. A few examples of known effects are given here.

Urban lighting

Air pollution

The urban heat island


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