Wednesday, 17 July 2013

Urban Heat Islands

Given that the Met Office has issued a heatwave "Level3" warning for the south-east of England it is appropriate to give some thought to how and why the conditions are particularly hot in cities, like London. Heatwaves are prolonged periods of excessively hot weather compared to the usual weather in the area and to temperatures normal for the season, so they are relative phenomenon.

Cities are warmer than the surrounding countryside due to human activities: the Urban Heat Island Effect (see the temperature map of London below to see why the term island is used). This effect happens because:

       Vertical structures behave like canyons, cooling winds and convection are reduced, slowing the heat transfer away from built-up areas.
       Heat absorbed during the day by concrete buildings, road surfaces…is released during the night, in the same way a storage heater does its job
       Heat is emitted directly from the large numbers of homes, workplaces and vehicles into comparatively small areas

Urban areas also dry very quickly, with drainage rapidly taking water away. This and having few green areas means less heat is lost by evaporation or transpiration by plants than in the countryside. Note Richmond Park’s effect in the images below (below, bottom left is a cooler purple area corresponding to the park, whereas the pattern either side is a belt of warmer area colour coded green).

We can observe that the bigger the city generally the bigger the urban heat island effect.  It is most apparent on clear and calm days and nights, particularly at night This is exactly the conditions that occur in hot weather.  A clear, calm summer London night is typically 6 °C warmer than in the surrounding countryside.

Air pollution factors compound problems. These days urban air pollution is associated with dry, hot sunny days in spring and summer (see image of LA below), rather than in winter (like the old London smogs were).

The main pollutants are now ozone, nitrogen dioxides and particulates and the main cause is now emissions from cars, lorries and other forms of transport that burn fossil fuels.Ground-level ozone is produced when the chemical emissions react in sunlight and in warm weather over several hours (see image below).
Ground-level ozone can be blown across large areas of the country and is not just in cities. Today, the most damaging pollution episodes occur during hot, dry, sunny weather and often accompany heatwaves. Pollution health impacts make heatwave health impacts much worse – and the biggest impacts are on the elderly and those with existing heart, lung and other health problems.

You can find advice on staying safe in the sun here.

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