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Local air circulation
The presence of tall densely packed buildings in cities changes the prevailing wind speed and direction. They also allow local air circulation patterns, for example the urban breeze, to be set up. Wind may improve the air quality in a city by clearing the air of pollutants but it also may cause too much heat to be lost from the buildings.
Air circulation in a city is controlled by natural and anthropogenic factors, for example, the air temperature, the roughness of the surface and the presence of various barriers (hills, forests, high buildings).
Urban areas warm up much faster during the day-time than non-urban area. This results in an atmospheric pressure difference between the two areas, low pressure over the city and higher pressure over the surrounding countryside. This pressure difference generates winds which blow into the centre of the city. These local winds occur over much shorter distances than those caused by atmospheric circulation patterns and air pressure differences over continents. When the wind in the city drops, the urban breeze can develop. As the air in the city warms up, it becomes less dense and it rises. As it rises, it spreads out and cools. As it cools, it becomes heavier and it sinks down over the suburbs. It then returns to the city and this cycle is known as the urban breeze.
Wind reaching the city changes direction. Streets with high buildings on each side of the road create tunnels for the wind to travel through and buildings perpendicular to the original wind direction change both the direction and the speed of the wind. Main roads leading into the city act as the main corridors by which the wind enters the city. In wide streets, the wind simply follows the direction of the street. In narrow streets the wind speed is significantly increased at street corners and local eddies are generated at squares and street junctions where different air currents meet.
Buildings act as barriers to the wind and, in the centre of a city, average wind speeds are about 20% lower than in the suburbs. Weak winds (wind speeds less than 3 m s-1) are seen more often in cities than in the surrounding countryside.
When the wind blows perpendicular to a row of buildings, the windward side is exposed to strong gusts of wind, while leeward side is in a so-called aerodynamic shadow. The strong gusts of wind often mean too much air gets blown into the building and this can have a negative impact on health and the comfort of the inhabitants. Local eddies develop on the leeward side of rows of flats and the size of the eddy increases with the height of the building. Decreasing the distances between the blocks of flats lowers the wind speed by up to 50%.
When the wind hits a high building, the air stream divides. A part of it moves upwards and the rest goes around the building. This causes an increase in the wind speed by upto 30% at the corners of the building. Lower buildings in the same area often suffer as a result of this modification in the wind direction. Air streams generated by the high buildings may, for example, cause the low buildings to vibrate.
Wind speeds greater than 3 m s-1 generally have a positive impact on the air quality in a city by improving the ventilation and increasing evaporation. However, these winds also disperse air pollutants to other regions and increase heat loss from buildings in winter.
About this page:
Authors: Sebastian Wypych, Anita Bokwa - Jagiellonian University - Cracow / Poland
Supporter: Mateusz Kaminski
1. Scientific reviewer: Prof. Barbara Obrebska-Starkel - Jagiellonian University - Cracow / Poland - 2003-06-20
2. Scientific reviewer: Dr. Marek Nowosad - Maria Curie-Sklodowska University - Lublin / Poland - 2003-06-16
last update: 2003-07-22