Climate in cities

Basics

Local air circulation

In a city, a local air circulation is formed. The so-called urban breeze may be observed, and high density of buildings modifies wind speed and direction. Wind may improve air quality in a city, but it may also cause a too intensive cooling of the buildings.

In a city, the air circulation is controlled by local natural and anthropogenic factors, e.g. air temperature, surface's roughness, presence of various barriers (hills, forests, high buildings). Urban area warms up much faster during the day-time than non-urban area. Therefore, a difference in atmospheric pressure develops, with an area of lower air pressure over the city and higher pressure over the surrounding non-urban areas. That difference generates winds blowing to the city's interior. Those are local winds, of different origin and much smaller spatial range than winds caused by atmospheric circulation and air pressure differences over a continent. When there is no wind (caused by the atmospheric circulation) over a certain area, the urban breeze is best developed. First the warmer air above the city rises and goes towards the suburbs as a so-called anti-breeze. There it cools down, gets heavier and sinks, and comes back to the city as a urban breeze.

1. Urban breeze
Author: Mateusz Kaminski 

Wind reaching the city changes its direction. It follows the tunnels created by the streets with high buildings on both sides, or omits the barriers consisting of the buildings placed perpendicular to the original wind direction. Main roads leading to the city are also main corridors with which the wind enters the city in the evenings. In wide streets, the wind just follows the street course. However, in narrow streets the wind speed is significantly increased at street corners. Moreover, the wind generates local eddies at squares and streets' junctions where a few air streams meet.

In a city centre, the wind speed is lower than in suburbs, and so is the consistence of the wind direction with the one outside a city. The wind speed is strictly correlated with the surface's roughness, i.e. density and heights of barriers causing the decrease of wind speed (buildings, trees etc.). In the city centre, the wind speed is reduced by 20% in comparison with the suburbs, and weak winds (< 3 m/s) are noted much  more often than outside the city.

When the wind blows perpendicular to the buildings located in a row, the windward side is exposed to strong gusts, while leeward side is in a so-called aerodynamic shadow. Moreover, the buildings are too intensively ventilated  which has negative impact on the health and comfort of the habitants. Blocks of flats standing one after another cause local eddies at the leeward side, but the size of an eddy increases with the height of a building. Decreasing distances between the blocks lowers the wind speed up to 50%. When the wind hits a high building, the stream of the air gets divided. A part of it moves upward, while the rest goes around the building, which causes the wind speed increase by 30% at the building corners. Low buildings located in the vicinity of the high ones, often experience the negative impact of wind direction's modification. The air streams generated by the high buildings (described above) may for example cause the vibrations of lower buildings.

3. Idealized flow in the vicinity of buildings
Author: Mateusz Kaminski

Wind speed higher than 3 m/s has mostly positive impact on aerosanitary conditions, as it improves the city ventilation and increases evaporation. But at the same time it enhances air pollution's dispersion, and in winter it causes losses of heat in buildings exposed often to the wind gusts.

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
educational reviewing:
last update: 2003-07-22