The climate in urban areas differs from that in neighboring rural areas, as a result of urban development. Urbanization greatly changes the form of the landscape, and also produces changes in an area's air.
Temperatures are higher in cities than the surrounding rural areas—this area is called the urban heat island. With rapid growth of megacities in the world, more sensible heat fluxes go to the atmosphere and thus making higher air temperature around urban area and its surrounding rural area. There are a number of causes of the urban heat island:
- Building materials have a lower specific heat capacity (the amount of energy that will heat a kilogram of a material by 1 °C) than grass and trees—the specific heat capacity of concrete is 800 Joules/kg whereas for soil it can be 2000 Joules/kg, so concrete heats up more quickly in the day, warming the air around it.
- Buildings are heated while the vehicles and air conditioning systems generate heat.
- Buildings act as a barrier to winds which would otherwise distribute heat and cool the city.
- Buildings and roads with dark surfaces have a lower albedo (reflectivity) and absorb more sunlight, becoming hotter. Sunlight not absorbed by buildings is mostly reflected into other buildings.
- Natural sources of water and humidity that can exert a positive cooling oasis effect are rare (subterranean, canalized urban streams)
The urban heat island effect tends to be stronger in winter because the colder air above the city is less able to rise by convection to allow the hot air inside the city to escape into the atmosphere. The effect is greater at night for the same reason.
Because cities are warmer, the hot air is more likely to rise and if it has a high humidity it will cause convectional rainfall – short intense bursts of rain and thunderstorms. Urban areas produce particles of dust (notably soot) and these act as hygroscopic nuclei which encourages rain production. Because of the warmer temperatures there is less snow in the city than surrounding areas.
Wind speeds are often lower in cities than the countryside because the buildings act as barriers (wind breaks). On the other hand, long streets with tall buildings can act as wind tunnels – winds funnelled down the street – and can be gusty as winds are channelled round buildings (eddying).
Cities usually have a lower relative humidity than the surrounding air because cities are hotter, and rainwater in cities is unable to be absorbed into the ground to be released into the air by evaporation, and transpiration does not occur because cities have little vegetation. Surface runoff is usually taken up directly into the subterranean sewage water system and thus vanishes from the surface immediately.
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