Li Yangxiang, a chief short-range weather forecaster with the Central Meteorological Observatory, focuses on the computer screen as she maps in isobars. [China Daily]

Li Yanxiang, a senior short-range weather forecaster with the Central Meteorological Observatory (CMO), fixes her eyes on the computer screen, as she maps in isobars.

Li, though unimpressive in stature, is one of the country's leading weather experts charged with the responsibility of telling the public what the weather will be like for the next few days in the most populous country on earth.

The data on the maps on her computer screen come from different sources. Once every three hours her colleagues in more than 2,300 local meteorological observatories all over the country feed in the basic data they have gathered.

In addition, she also gets input from satellites and radar stations.

For the past two months, Li and her many colleagues have been spending most of their time in the weather consulting room.

They are watching over the flooding in the Huaihe River basin, the most severe the basin has experienced since 1954, as well as tracking the hot and dry weather that has been plaguing a large part of the country south of the Yangtze River for almost six weeks.

On some days she has to start work at 4 in the morning.

"This is the most crucial time of the year for us," Li said.

On July 23, her shift stretched well into the small hours of the next morning, as typhoon Imbudo was starting to be felt in South China.

"When typhoons arrive, it is common for us to work shifts of more than 10 hours," Li said.

So far, three typhoons - Swan, Imbudo and Morakot - have hit the southern and eastern parts of the country, bringing a breath of fresh air to some areas that have been in the grip of drought for weeks.

Heatwave

However much Li and her colleagues had been hoping for fronts bringing rain to areas south of the Yangtze River in the provinces of Central and East China, they had to wait for six weeks before they finally saw a gleam of hope last Saturday.

Areas south of the Yangtze have been plagued with dry spells and heatwaves since late June due to sustained high temperatures and little rainfall.

The rain belt held stationary in the Huaihe River valley between late June and early July brought heavy rainfall and caused catastrophic flooding in the region. Some 840,000 people in Henan, Anhui and Jiangsu provinces had to be evacuated from their homes and the lives of more than 47 million people were affected.

Meanwhile, the areas south of the Yangtze River valley, especially parts of Central China's Hunan Province, East China's Zhejiang, Jiangxi and Fujian provinces and Shanghai, as well as South China's Guangxi Zhuang Autonomous Region, began experiencing sustained heatwaves starting in early July.

After July 20, the rain belt shifted northward into North China. As a result, the areas reporting high temperatures further expanded into Henan and the northern parts of Anhui and Jiangsu provinces.

What Li and her colleagues have been observing is what they term a sub-tropical high. Li and her colleagues have discovered that these vital meteorological conditions influencing the movement of the rain belt have remained abnormally strong and stable this year.

This sub-tropical high-pressure system is a region of relatively high pressure that lies between 25.0 and 35.0 degrees north latitude. In the Northern Hemisphere such high-pressure systems are characterized by clockwise wind patterns moving around high-pressure centres and subsidence (air sinking from aloft), with relatively low humidity and generally clear skies.

"The system initiates heat waves wherever it moves," said Yao Xuexiang, deputy director of CMO.

Sunny days have continued in areas controlled by the sub-tropical high. Heat has poured in constantly, with temperatures climbing from 30-odd degrees Celsius up to as much as 43 C.

Many cities in the south have seen old high temperature records broken. For instance, by August 2, 40, or 68 per cent of the local weather stations in East China's Zhejiang Province, had reported record high daily temperatures above 40 C.

The continuing heatwave in the south shattered records going back 50 to more than 100 years in Shanghai, Chongqing, Wuhan, Fuzhou and Changsha.

Li and Yao and their colleagues have forecast and watched three typhoons land in the southern part of China and have also been watching for any southern drift of cold northern air.

Those regions under the heat spell might escape from the hands of the sub-tropical high when typhoons land or cold northern air drifts into the south, Yao said.

However, the effect of typhoon Swan was mild, reaching only the south tip of South China and having little effect.

The second typhoon, Imbudo, was much stronger, but it landed in western coastal Guangdong, later moving into the Guangxi Zhuang Autonomous Region.

Tang Zhengwu, 40, holds some withered rice as the result of the heatwave. [China Daily]

Following the first two, tropical storm Morakot hit Jinjiang in Fujian Province on August 4. The storm crossed Taiwan, then the Taiwan Straits, and finally landed in Fujian, but it had already lost most of its energy and was not as powerful as before.

It brought rainstorms in some regions of Fujian, Guangdong, southern Jiangxi and southern Zhejiang.

Meanwhile, the cold air front from the north built up and drifted south, with rain falling in its wake from last Saturday. "Now the extent of high temperature areas is shrinking, and heat intensity is weakening," said Yao.

Causes

China is not the only country suffering from heatwaves this year.

In Europe, heatwaves that have continued for some two weeks have already caused huge forest fires in Portugal, Spain and Italy.

According to news agencies, medical experts have claimed that some 50 people in France have died as a result of the smothering heat.

On Sunday, temperatures in Britain broke an all-time record.

Yao attributed the historically rare weather to the abnormal movement of the upper atmosphere.

However, forecasters are still trying to ascertain more exactly why the extremes of heat have appeared this year.

"We could only make some medium-range and short-range forecasts for the present extremes of summer heat," Yao said.

But he noted that global warming, the deterioration of the natural environment - the depletion of forests, for instance - and the process of urbanization have somehow enabled the destructive power of severe weather to grow stronger.

"Planting more trees in cities and returning grassland to forests will help to alleviate the high temperature effect," said Yao. Shortages of drinking water in the heatwave-affected regions were clearly the result of excessive consumption of water resources in the past, Yao added.

If changes are not made, he said, we are likely to see more abnormal weather in the future.

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