This summer brought unprecedented extreme weather to Beijing. The worst storm in 20 years hit the capital on July 10. A dust whirlwind, the most severe of its kind seen in 55 years, devastated the construction site for the National Swimming Center on August 27. It left 44 workers injured and two dead. In addition, many other short-lived storms have also visited the capital this summer.

According to the meteorologists, there is a connection between the extreme weather and city planning considerations. This summer's extreme weather has been notable for four characteristics. All represent unprecedented phenomena:

·         When extreme weather came, it did not last long. The storm of July 10 lasted less than 3 hours. The whirlwind on August 27 was over in 20 minutes.

·         It would come and leave, both suddenly and unexpectedly. Its unpredictability made it impossible for people to take steps to prepare for it.

·         The extreme effects have been localized. Beijing has a built-up area of some 1,600 square kilometers, but the storm of July 10 focused itself on an area of less than 100 square kilometers. Some 100mm of rain fell in the northwest while the rain barely made the ground wet in the south of city. On August 27, the whirlwind only affected an area of several hundred square meters.

·         The weather has brought unprecedented losses and disruption. The freak whirlwind brought death and injury to the city on August 27. The storm of July 10 only lasted three hours but it brought the capital to a standstill with a serious traffic jam.

Although the extreme weather is the result of quite a number of different factors such as cold air and convection currents, city planning and construction do exert an important influence on the local weather.

Meteorological observations reveal that heavy storms and high winds are closely related to the city's "micro-climate". Large areas of concrete or asphalt road surface produce localized "heat islands" within the urban environment. This summer, temperatures of some 80 Celsius were being recorded on concrete and asphalt road surfaces. Meanwhile neighboring areas of greenery were only about 40 Celsius.

Such large temperature variations in close proximity make localized extreme weather conditions more likely. Meteorologists have pointed out that when the temperature of concrete and asphalt roads reaches 80 Celsius, it can have a significant impact on rainfall. This is why Beijing residents could find flooding to one side of a road but just a little rain on the other.

In addition, there is what is known as the "valley effect". This is becoming an increasingly apparent feature of Beijing's townscape. When the city as a whole encounters winds of say force 6 or 7, they are concentrated in the "valleys" between high buildings. Here the wind can briefly touch force 12. This is Typhoon level! It is hardly surprising that so many billboards have been blown down this summer.

Meteorologists suggest that greater consideration should be given to avoiding localized extreme weather during the planning phase. It would be helpful to increase large-scale tree-planting and grassed areas and reduce the area of road surface. High buildings should be carefully laid out in relation to each other and built further apart. City planning should allow for the provision of "urban wind paths" to proactively control how the wind will move through a built-up area. Not only can these improve air quality by quickly removing pollutants, but they can also benefit the urban "micro-climate" reducing the chances of extreme weather.

(China.org.cn by Wang Qian, September 5, 2004)