PDS notes: URBAN CLIMATE: WINDS
Wind velocity is reduced by buildings as they create frictional drag and act as wind breaks. The mean
annual velocities in urban areas are up to 30% lower than rural areas and periods of calm are 10-20%
more frequent.
Tall buildings channel winds down urban ‘canyons’ (The Venturi Effect).This is most noticeable when the
buildings are aligned parallel to the prevailing wind.
Local thermals that occur because of the urban heat island effect can cause small-scale turbulence and
eddies. The heat island effect can also give rise to ‘country breezes’. On calm clear nights when the heat
island is at its maximum there is a surface inflow of cool air towards the highest temperature (low
pressure). The winds are at a very low velocity and are quickly decelerated by surface friction.
Buildings above 25-30m.above ground level can create unpleasant wind conditions and velocities can be
greater than those in the countryside. Wind speed increases with height above ground level and tall
buildings experience much faster winds nearer the top. These faster winds are deflected by the buildings
downwards and create gusts at ground level.
If there are openings or passageways underneath buildings then winds may be sucked through as
‘throughflow’ into the zone of low pressure which develops on the leeward side.
The effect of buildings on winds can lead to several consequences;
1.Reduced chill factor.
2.Lack of adequate ventilation at ground level so car pollution is not disturbed.
3.Gusts around tall buildings can result in injury are discomfort e.g. water sprayed from fountains.
4.Gusts and eddies swirl around dust and litter.
Problems caused by winds can be reduced with careful planning.
1.Not erect buildings above 25-30m. high (impracticable).
2. Zone land use around buildings.
3.Design buildings so that the downdraught does not reach the ground (see sheet).
4.Erect canopy over walkways of buildings already erected.
5.Put up baffling around building sites to prevent dust being blown around.
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6.wash down the wheels of lorries leaving building sites.
Winds/ Air movement/Flow of air
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Urban heat-islands can create own winds. Higher temperatures = lower pressure over cities which
draws in air from the surroundings.
Uneven surface of urban area creates friction and reduces mean wind speeds.
Buildings can provide shelter-incidence of extreme gusts lower and frequency of calms greater – at
right angles to wind direction.
Winds can be concentrated between buildings- increasing speed and powerful gusts in some streets
parallel to wind direction.
Tall buildings intercept the faster flow of air above the ground and deflect some downwards creating
severe gusts around some tall buildings.
Winds are affected by the size and shape of buildings
Tall buildings
A single building displaces air above and around the sides of the building and it is also pushed
downwards in the lee of the structure.
The air pushes against the side of the building with relatively high pressure. Main force hits at 60% of
height.
On the leeward side the air coming over the top of the building pushes the air down – forming an eddy
(vortex) at ground level.
The air also sweeps around the corners of the building causes high winds on the corners.
(The eddy ( vortex) is worse if there is a small building next to the tall building as the air is trapped. )
Two buildings
The effect of more than one building depends on the spacing of the buildings.
Close together and the wind skims over the top- eddies in-between the buildings caused by the lower
pressure.
If the buildings are not quite so close together the flow of air is less smooth when it reaches the second
building and there are more pronounced eddies
Urban Winds – missing words
The average annual wind speed over urban areas is lower than that over rural areas. This is because
……………………………………………………………………………………………………………….……………
…………………………………………………………………………………………………………………………....
Deep narrow streets that are at right angles to the prevailing wind are sheltered from the wind and have
frequent calms because ……………………………………………………………………………………………….
Some areas of cities can experience severe gusts. This can happen when buildings are aligned with the
prevailing wind.
This is because ……………………………………………………………………..………..……..
Also there are high winds around the corners of tall buildings because ………………………………………..
There are also high gusty winds at ground level on the windward side of tall buildings because……………
If buildings are arranged close together the winds ……………………………………………………………...….
If buildings are well spaced they act as ……………………………………………………………………………
If the buildings are fairly close together then………………………………………………………………………...
On calm, clear nights anabatic winds can be produced. This is when ………………………………………..….
The effect of wind speed speed/gusts can be reduced by …………………………………………………………