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Storms And The City

By Alex Campbell on July 8, 2011 in Other

Large urban centres like Sydney have profound impacts on the local environment, but did you know that our big city home is also affecting our weather? The air directly above cities is significantly hotter than nearby air blanketing rural or undisturbed habitats. This is because buildings and other city infrastructure are made with materials like concrete and steel that store large amounts of heat.

Worldwide, cities are growing rapidly and becoming increasingly dense with fewer and smaller ‘green’ areas, so the stored heat builds up during the day and keeps the temperature a few degrees higher than it should be overnight. Scientists have been aware of this phenomenon, called ‘urban heat islands’, for at least a couple of hundred years and it’s known to occur in both summer and winter.

Interestingly, brand new research has demonstrated that as well as changing the ambient temperature, cities can also influence the pattern and intensity of thunderstorms. From beneath, thunderstorms appear chaotic: swirling, writhing masses of dark clouds punctuated with lightning rods and thunder claps. But from above, storms behave in a rather organised fashion. Meteorologists can get this view by monitoring radar and satellite images of storms, which are made-up of cells. Storm cells are areas of low pressure created by the updraft of hot, moisture-rich air travelling from the Earth’s surface to the storm clouds above. The cells tend to line up behind one another, or form a band-like structure and move over the country-side more or less predictably in this arrangement, until they encounter a city.

The extra heat retained by cityscapes, tall buildings reaching into the clouds and a high load of particulate matter in the dirty, city air, all seem to upset thunderstorms, causing the cells to split-up as they skirt around the urban jungle. Often the storm cells regroup downwind, forming more intense versions of their former selves.

Researchers from the U.S.A. studied thunderstorms in the state of Indiana over a 10-year period and found that more than 60% of the daytime storms changed their behaviour as they encountered the large metropolis, Indianapolis. In comparison, storms that passed over rural areas less than 100km away continued to behave predictably. They also built mathematical models that simulate storms moving to demonstrate graphically how the urban landscapes can interfere with the ‘normal’ behaviour of the storm cells and confirm that cities can disrupt and intensify thunderstorms.

The researchers are now busy developing mathematical models in which further changes to landscapes can be simulated to see how thunderstorms and other weather patterns might be affected. They hope that their work will help improve predictions about severe weather and flooding. So the way we use and abuse the land can have big impacts in our atmosphere, which can quite literally come thundering back down upon us.

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