Saturday 31 March 2012

UHI's (Urban Heat Islands) Winds

Specification: Winds: the effects of urban structures and layout on variations in wind speed, direction and frequency.


The geometric features of urban structures and their layouts have major effects upon wind speed, direction and frequency.


There are three main types of effects of wind within urban areas:

  • Due to the variance in building height within the urban canopy layer, buildings can exert major frictional drag upon wind speed which in turn creates turbulence and gives about abrupt changes in both direction and speed of the wind. Ultimately, this slows the average wind speeds within urban areas in comparison to rural areas which might see higher velocity winds.
  • Where the geometric features of tall buildings can slow wind speeds, they can too channel airflow into urban canyons beneath them. This leads to an increase in wind speed as a result of the overpressure of airflow between buildings (effect is known as the Venturi effect) and can increase the frequency of turbulent winds.
  • As a result of temperature increases within cities, convection can create localised low pressure cells within urban areas in contrast to rural areas which can influence surface inflow from colder rural areas into the urban cities. This can led to a greater frequency and increase to wind speed.
As mentioned in the above points, the size and shape (geometry) of buildings can affect wind speed, its direction and its frequency. As shown within the photo below, the obstruction of a building can cause a modification to airflow passing over it. Air is displaced upward, around the sides and then pushed downwards in the lee side (downwind) of the building.

Urban structures modify airflow around it causing eddying of vortices on the lee side and forcing airflow upward and around the buildings.


In reference to the windward side of the building, air pushes against the walls of the structure with high pressure. Air which then flows around these sides of a building forms vortex's as the airflow reaches the ground and sweeps into windward corners. Essentially, the presence of urban structures cause these abrupt changes in wind direction. Furthermore, the frequency of these changes is then decided by the density of buildings with the urban area (which will be discussed in the below section).

On the lee side of the building, a zone of lower pressure causes vortex formation behind it which are essentially turbulent winds that can cause building damage and strong winds that may knock pedestrians off their feet. If two buildings share airflow between them, they can become subject to the Venturi effect. This is where the overpressure in the gap between the buildings on the windward side causes increases to wind speed which can reach extremely high velocities. It is for these reasons, some urban layouts are planned with buildings spaced far apart with a large gap to prevent this overpressure of airflow. Furthermore, some buildings have used stilts to attempt to offset the Venturi effect which can cause major damage to buildings. However, the way in which urban layouts are planned is important to maintain a sustained airflow to remove surface level pollution from vehicle exhausts.

In most cases, buildings are narrowly spaced and so airflow between them is interfered and wind speed is increased. However, the impact of this is dependent upon the height of the buildings and the amount of space between them. Widely spaced buildings can act as single buildings/isolated blocks however in contrast, as the gap decreases between buildings, airflow becomes more and more likely to become subject to overpressure and thus the Venturi effect which can change the frequencies of airflow by producing a complex pattern of changes in wind direction and speed.


Often, the urban layout is structured in such a way that winds speeds are often abrupt and turbulent as a result of frictional drag. Furthermore, the modifications to airflow by urban structures and the potential Venturi effect caused by this means there are vast variations in wind speed direction, velocity and frequency  as a result of geometric features of urban areas. As a means to prevent pollution integrating within the urban canyons of airflow, pollution emitting chimneys are often situated high enough so pollutants released are released into an undisturbed flow above the buildings. This means pollutants are not emitted into the downward lee-side eddying of vortex, air flowing around the sides or downward flowing air which circulates on ground level in vortices and within general airflow and can act to cause respiratory problems, corrosion of buildings, damage to ecologies (especially if pollutants are present within the downwind urban plume), haze, acid rain and the soiling of buildings depending upon the pollutants involved.

Alas, the end of Urban Heat Islands.

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