Wind speed patterns in a city with buildings

In nature, wind patterns at ground level are influenced by trees, hills and mountains. These objects have an individual effect on the surrounding area, and they also have broader effects when found in groups, such as forests and ridges. In a similar fashion, buildings affect the wind in urban locations, but consider they have sharp corners and rectangular shapes not found in nature. The shape of buildings can cause localised wind acceleration or turbulence: in severe cases, the  wind can cause pedestrians to trip over, or it can fling objects at high speed.

The effects of buildings on urban wind patterns can be analysed individually or in groups. Individually, buildings cause the downdraught effect, which makes the surrounding streets and sidewalks windier. In groups, buildings can form “street canyons”, which are roads that have many tall buildings on each side.

Wind monitoring plays a key role in any attempt to reduce draughtiness and turbulence from city streets, making them safer for pedestrians and drivers. Also consider all the activities performed in streets in addition to commuting, such as public works, product deliveries and commerce. A reliable wind monitoring system can be used to analyse the existing condition, and also to assess the effectiveness of any measures proposed.

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The downdraught effect: how high-rise buildings affect the wind

When wind reaches the wall of a building, it gets deflected in all directions. Some of the wind is deflected upwards and around the sides of the building, causing no effect at ground level. However, a significant portion of the air is deflected downward along the building wall, causing draughtiness and turbulence at ground level.

In general, the downdraught effect becomes more severe as the height of a skyscraper increases, since a larger wall deflects more wind downward. Given that streets do not vary much in width, more air deflection results in higher speed and turbulence – a larger air volume is displaced in the same space, so it must move faster.

The angle at which the wind hits a building has a strong influence on the downdraught effect. In general, the effect is more pronounced when the wind hits a building wall head-on, becoming much less severe when wind approaches at an angle: more air is deflected to the sides and not downward.

Wind patterns in street canyons

As previously explained, street canyons are formed when parallel groups of buildings to the sides of a road resemble a natural canyon. This creates a disruption of wind patterns, which depends on the angle at which the wind reaches the street canyon.

    If the wind is parallel to the direction of the street canyon, it accelerates as it is confined to a reduced space.

    If the wind is perpendicular, it creates a large pocket of turbulence in the air volume contained between the two groups of buildings. These vortices are very detrimental for air quality, since they trap vehicle emissions in street canyons.

    Combinations of both effects may occur when the wind hits the street canyon at an angle: acceleration along the canyon direction, along with turbulence.

The way in which these effects present themselves is influenced by the dimensions of the street canyon: the buildings to the sides determine its height, the street determines its width, and both features determine the canyon length.

How to control wind disruption from buildings?

There are two ways to address urban wind speed and turbulence: prevention and mitigation. To prevent excessive wind speed and turbulence, buildings can be designed to minimise the downdraught effect, while large-scale urban planning can be oriented towards minimising street canyons.

Wind disruption can be controlled in new constructions or urban developments, by designing building geometries that reduce downdraught. However, this is impractical in existing buildings: modifications to building shape are prohibitively expensive. In these cases, the best approach is using landscaping features to slow down strong winds – planting trees in walkways is an simple and effective solution.

Conclusion

Consider that cities are not static: new areas are constantly being urbanised, while existing buildings are demolished or renovated. This creates an ongoing opportunity to deploy wind monitoring, in order to improve wind conditions for drivers and pedestrians.

Wind patterns themselves are becoming more erratic due to climate change, and extreme weather events are more difficult to predict – consider the storms that hit the Caribbean and the British Isles in 2017. The risks associated with strong and turbulent winds are increased in urban settings, due to the abundance of objects and debris that can become airborne at high speeds. This provides even more reason to monitor urban wind patterns. Keeping the wind under watch is especially important for construction companies using tower cranes in urban settings, since a crane collapse can cause significant collateral damage.