A 30-MINUTE thunderstorm on July 12 in Shanghai killed seven people and injured more than 40 as well as damaging hundreds of houses, vehicles and trees and causing electricity blackouts.

"It is rare to see such a severe storm with high winds covering almost the whole city," according to Zhang Zhenyu, spokesman for Shanghai's Flood Control Headquarters.

The rainfall in some districts such as Yangpu and Huangpu was between 29mm and 32mm. In other areas, it ranged from 3mm to 23mm.

"The rainfall wasn't too heavy but due to its intensity - the storm was over in half an hour - it seems to have been rather severe," he said. Although the rainfall was comparatively light, the wind velocity was gale force, measuring up to Force 9 in some parts of the city.

Dai Xinfu, chief weather forecaster with Shanghai's Central Meteorological Observatory said the severity of the storm was due to a collision between cold and warm air masses over the city. Continuous high temperatures for three days had heated the air above the city which was then hit by a large cold air mass moving down from the north-west.

More storms, although not of similar strength, are expected over the summer months. The city will be given an alarm signal of potentially catastrophic weather about half an hour before a storm breaks. "Because of some shortcomings in the technology we can only make a qualitative prediction and cannot offer a detailed analysis of rainfall," Dai said.

Uncertainty about the strength of storms and where they will do most damage is characteristic of this type of summer weather.

Calamitous rain

In last Monday's storm, most of those killed were migrant workers crushed under collapsed buildings.

At about 6:00pm, high winds blew down a tower crane at a construction site on Cao'an Lu which fell across the roofs of more than 50 houses and killed one person.

At the same time, the winds sweeping through Lujiazui in Pudong District knocked down a stall in a construction site, injuring six, one of whom later died. Flying debris damaged more than 10 cars parked nearby.

A collapsed factory in Changning District killed two people and injured one. Putuo and Qingpu districts each reported one resident killed during the storm.

The storm also affected civil aviation in the air over the city. Passengers on a jet about to land at Hongqiao International Airport were shaken by air turbulence and seven of them, plus three stewards, were severely injured. Four are still in hospital.

"Shanghai has never seen so many people killed and injured by a storm in recent years," Zhang said. "In 1991, 10 people died during a storm. But over half of them died from electric shock, not because they were crushed by falling buildings.

"It is a pity that most of the victims were migrant workers," Zhang said. "Lacking knowledge of how to take proper safety precautions, most of them chose the wrong place to escape the wind and rain. They did not know that high walls would fail to protect them but could instead kill them in a severe thunderstorm."

Jiao Yang, spokeswoman for the Shanghai Municipality, said last Tuesday that work at all construction sites in the city must stop whenever the local meteorology department issued a weather alarm.

Keeping from dangers

Experts warned that in bad storms with heavy rain and high winds, residents are better off staying inside. They should switch off electrical household appliances in the kitchen and the television and telephone.

People should not shelter under high buildings because of the danger that parts of the building may become dislodged and fall on them.

"The city government is also launching an examination of large advertising billboards around Shanghai," Zhang said. The city's administration is considering a regulation to limit the height of advertisements. Those found to be too high would be removed gradually.

Guo Hua, an official with the city's Environmental Protection Bureau, said the bureau has already notified all billboard advertisers to conduct safety checks on their outdoor advertisements which must now be of a construction standard to withstand winds up to Force 12.

"Another big problem is outside air-conditioners," Zhang said. "Shanghai has many air-conditioners that have been hanging outside apartments for over 10 years. Actually, they can become dangerous if they have been there for just five years."

But how to ensure storm safety - which involves private property - is an administrative headache.

"With summer advancing, we have to strengthen examination procedures concerning potentially dangerous buildings and tower cranes to eradicate the concealed dangers they represent," Zhang said.