Downdraughts and eddies in city streets
THROUGHOUT the world, cities have been growing at an alarming rate.
As they grow, they have caused changes in climatic conditions. Nearly every city has skyscrapers, with many a city claiming to have constructed the tallest building in their nation or even the world.
This upward growth in building-height is to maximise floor space in central city areas, where high land-rent values exist.
Gradually, the proliferation of high rise buildings has presented architects, engineers, planners and climatologists with evermore complex problems.
The interaction between tall buildings and the surrounding atmosphere has been viewed in terms of wind loading and thus engineers have thought long and hard to find solutions to the enormous wind pressures exerted on buildings at high levels.
As wind speeds increase with altitude, tall buildings encounter increasingly strong winds up to height of between 500 and 1,000 metres.
Attention has become increasingly focussed on the comfort and safety of pedestrians in proximity to tower and slab blocks.
Clearly cities built in different climatic zones in the world require different approaches. In middle and high latitudes, shelter from the wind is important whereas in the subtropics and tropics, maximum ventilation is a prerequisite.
Our physiological response to the atmosphere is complicated by the types of clothing we wear, the nature of our activity and our respective age.
The most recent research into the microclimate of city centres has focussed on the wind environment for pedestrians, in order to make it comfortable for them. Beaufort wind scale
We can, without knowing it, determine the force of a wind on the Beaufort scale. This is an internationally accepted wind scale, invented by the British Admiral Beaufort in 1805. Primarily designed for mariners, it is often used on shipping weather forecasts even today.
Above Force 3 (five metres per second wind speed), people feel uncomfortable for many activities. We are all familiar with the discomfort of excessively gusty and strong winds around the base of tall buildings. One needs to persevere in trying to open an umbrella in the city streets on windy days unless one is aware of the building’s aerodynamics.
Windy conditions can present real hazards to pedestrians. The pressure exerted by the wind increases with the square of the wind speed and a doubling of the wind speed around the corner of a building causes a fourfold increase in pressure upon pedestrians.
Elderly people, the infirm, and young children find it hard to withstand sudden changes in wind direction and the resultant increases of pressure on their bodies and may be blown over.
Local shopkeepers whose businesses have been located in low level shop lots for many a year may be affected by high rise developments near them, usually related to inner city regeneration projects, in the form of apartment or office tower blocks. The swirling winds may drive customers away.
Are such microclimates the unavoidable product of urbanisation?
In a city’s suburbs or in small towns generally buildings are almost uniform in height, thus keeping the street-level environment pedestrian safe – apart from road traffic.
As soon as buildings rise up to 25 to 30 metres, the wind speeds increase.
Faster wind speeds are partly deflected around the top of a high rise building, and the rest being deflected downwards as downdraughts along the face of the building, causing eddies to develop at pavement level. This can be seen by the swirls of litter in streets.
With an opening or passageway under a tall building, winds are sucked through as ‘throughflow’. Actually, plant life is also affected as shrubs and trees in ornamental flower beds bend much the same as they do on wind exposed coastal areas or on mountains.
Even in Singapore, I have noticed such partially misshapen plants. Sadly, developers’ priorities overlook the man in the street, for who cares about urban ‘whirlwinds’? Many urban developments happen without any consultation with meteorologists and aerodynamic specialists, relying solely on the developer’s purse and net return and an architect’s imaginative design.
Looking at high rise apartment buildings around the world, I frequently wonder why ‘infinity’ swimming pools are located on the roof of such towering blocks and why outdoor basketball and tennis courts are located at the foot of these buildings for in both areas wind speeds are strongest unless sports facilities are located on the leeside.
Although wind tunnel experiments for future tall building design are expensive to run, they do save developers much money in correcting their designs and avoiding expensive alterations in hindsight after a building has been constructed.
Reinforced vertical glass windbreak screens and overhead canopies are costly additions and work well in deflecting the descending wind shears from tall buildings.
Equally, a tall square building should never face the prevailing wind for fear of winds being deflected around its right angled edges, thus affecting pedestrians. Certainly, this aspect of urban microclimatology is worth exploring to avoid wind eddies, with dust and litter swirling around our streets but, moreover to keep vulnerable citizens safe.
Whilst visiting, on windy days, central city areas in London, Singapore, and Kuala Lumpur, I have felt the tall buildings’ downdraughts and seen eddies of dust.
Fortunately, in Kuching, Miri and Kota Kinabalu, because of the relative spacing of high rise buildings, such whirlwinds do not occur.
May urban planners, architects, and local councils not be tempted to build high in the sky or such buildings close to each other. Please, do bear in mind the knowledge of local microclimatic and aerodynamic experts, for the benefit of daily pedestrians and nearby shopkeepers, restaurateurs and our coffee shop owners.
The forces of nature can be overcome and in the very technological 21st century we should adapt these technological achievements and advances to the benefit of mankind, now, and for the future. We need to think beyond the present.
Photo shows tower blocks in Singapore.