The weather is obviously not uniform across the country, and there can be big differences in just a short distance. By building large cities in what once was a rural setting, we can disturb the weather and give the location its own micro-climate. This is called an urban heat island.
City planners are being warned about the problem of 'urban heat islands' and are being urged to consider the problem when allowing developments in the city. For a number of years researchers have identified the problem - the buildings and roads act like giant storage heaters.
The large amounts of concrete, asphalt and bricks used in buildings and roads 'soak up' heat in the daytime and store it. The energy is then released during the night time. In addition, a lack of natural environment in the city that absorbs less heat than buildings, such as open spaces, has an impact. Heat released from vehicles and places like factories also add to the heat problem.
The effect is an increase in temperature and that often means a massive difference between London and its surrounding areas. On some days an increase of around ten degrees has been known. This is illustrated on infrared satellite images where in the summer in particular, the major cities are seen as much darker areas in comparison with the rest of the country.
Another reason why large towns and cities are generally warmer is due to decreased amounts of evaporation. In the countryside there is more water. As the water evaporates the process of changing from a liquid to a gas uses latent heat, which cools the surroundings. Roads and pavements do not retain moisture as fields and vegetation do, adding to the warming effect. You can easily feel this by standing on a black tarmac road on a hot summer's day, and then walking onto some parkland. You will immediately feel cooler.
In America, researchers at N.A.S.A. have been studying the effect and they say that some cities are so hot they can even generate their own weather, such as causing violent thunder and lightning storms.
Certainly in London we see less rainfall and snowfall as well as higher temperatures. Experts are also concerned that heat islands can cause pollution problems, with high ozone levels being recorded in a number of places.
Higher temperatures react more with sunlight and car exhaust fumes to produce the poisonous gas ozone. This can reach dangerous levels in some larger cities. Some people are worried that this problem will increase, with other cities around the country creating their own heat islands as they develop their business and residential areas. It's hoped they can perhaps think about allowing lower building and leaving open spaces alone.
In London, planners are being urged to re-think how they develop so the situation doesn't worsen. There is a good argument for enlarging parks and gardens in cities, planting more trees and creating larger lakes, not just for the pleasure to look at and relax by, but to try and cut down on the soaring temperatures, increasing pollution levels and the need for even more air conditioning, which adds to global warming. The whole issue of urban heat islands is being looked at along with global warming and the effect humans have on our weather and climate in the long-term.
At the rate the problem is increasing, we are likely to see temperatures rise even further in the next fifty to a hundred years.
Posted on 25th July 2006
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