In his article "Research on the Distribution and Reasons of Dusty and Sandy Weather in Chinese History," published in the 12th issue of the Chinese magazine "Xinhua Wenzhai (digest)," Wang noted that dusty and sandy weather is far from being a new disaster in China. Historical records indicate there have been many serious sand storms sweeping across the country. China Daily staff reporter Liu Jun reports Wang's views.

In the Eastern Han Dynasty (25-220), historian Ban Gu set aside the "Wuxingzhi (Records of Five Elements)" in his work "Hanshu (History of Han)" to record the disasters and strange phenomena occurring from the Spring and Autumn Period (BC 770-BC 476) down to the end of the Western Han Dynasty (BC 206-AD 24).

Following this tradition, most works of Chinese dynastic histories called the "Twenty-Four Histories" have recorded natural disasters. Traditionally, the "Twenty-Four Histories" have taken up the authoritative position in the recording of Chinese history. Thus Wang mainly based his research on these annals.

According to modern meteorology, dusty and sandy weather can be roughly put into three ascending categories.

Floating dust happens with or without little wind. The dust is either carried to the present location by high-altitude air currents or is suspended evenly in the air after a sand storm. The visibility when floating dust occurs is less than 10 kilometres. The sun appears white and any distant scenery is yellow brown.

The next level is floating sand, which is blown into the air by strong wind. The air is rather indistinct and visibility is between 1-10 kilometres.

The highest level is of course the sand storm. The powerful wind takes ground sand and dust into the air, lowering visibility to less than one kilometre. The sky appears dusty brown or even reddish brown.

Wang found that in Chinese annals, there have been very specific recordings of dusty and sandy weather.

For example, in the "Records of Five Elements," Ban Gu said that the northwestern sky seemed to have caught fire one night. The next morning, powerful winds came from the northwest. The wind appeared reddish brown and quickly filled up every corner of the sky. Throughout the day and night, yellow dust fell on the earth like rain.

Among the recordings, Wang found several appalling notes about the aftermath of the disasters.

In the "Songshi (History of Song)," it is said that in the year 1007 under the reign of Emperor Zhenzong in the Northern Song Dynasty (960-1127), a strong wind brought yellow dust that shrouded the sky. The farm fields around the capital were destroyed.

The "Mingshi (History of Ming)" records that in 1590 during the Ming Dynasty(1368-1644), a wind storm carrying sand and dust lasted from morning till night. Numerous houses were devastated.

The most difficult thing in studying the historical records of disastrous weather was finding out the precise location of the disaster, Wang said. About half of the weather records in the "Records of Five Elements" of the "Twenty-Four Histories" are vague in the locations of the climatic disasters.

In ancient China, everything abnormal in nature was seen as some sort of hint from heaven. These phenomena had to be reported to the Emperor and recorded by historians.

For the reports sent by local officials, a precise location was usually indicated. But when historians directly noted disasters that took place near the nation's capital, they normally omitted the location.

Thus Wang treated all the records without a location as meaning a place near the dynasty's capital. Through this, he was able to find out the geological changes of the dusty and sandy weather in Chinese history.

Cycle of sand storms

From the 4th century BC until 1900, Wang put the occurrence of dusty and sandy weather in China into four periods.

"Observed on a time scale, the occurrence of such disastrous weather is becoming more frequent, serious, longer and extensive," said Wang.

There were only two records about the dusty and sandy weather before the 4th century BC. From the 4th till the 10th centuries, 39 such disasters happened. The figure grew to 97 between the 11th and 15th centuries, then 115 in the 400 years before 1900.

In the spring of 1677 during the rule of Emperor Kangxi of the Qing Dynasty (1644-1911), a sand storm lasted for over 40 days. Such a record is rather rare, Wang noted.

While the general trend of dusty weather is increasing, Wang pointed out that there have been five peak sand storms between valley periods.

It is worth noticing that before 1051, there had only been five cases of dusty weather within a time span of 50 years. But from 1051 till 1100 within the Northern Song Dynasty, the number sharply rose to 13. From 1151 till 1200 during the Southern Song Dynasty, an astounding number of 27 sand storms took place, more than that in any 50-year spans between the 4th century BC and 1900.

In contrast, from 1368 till 1463 during the Ming Dynasty, Wang didn't find any records of dusty and sandy weather.

As for the seasons in which dusty and sandy storms took place the most frequently, spring took up 71 per cent among all the 252 records with clear seasonal indications. Winter took up only 7 per cent, while summer and autumn together took up 15 per cent.

Apart from time, space is another element Wang has focused his studies on.

While dusty and sandy weather visited the vast region to the north of the Yangtze River drainage area, the Yellow River and Haihe River drainage areas had been the main stage for such destructive forces of nature.

As the sandy wind blew stronger, it also affected more places in China. Before the 10th century, the middle and upper reaches of the Yellow River often witnessed sand storms.

Between the 11th and 14th centuries, the location of sandy weather gradually moved eastward. The area to the east of today's Xi'an (the capital of Northwest China's Shaanxi Province), west of Shijiazhuang (the capital of North China's Hebei Province), north of Zhengzhou (the capital of Central China's Henan Province) and south of Hohhot (the capital of North China's Inner Mongolia Autonomous Region) was frequented by the most sandy catastrophes.

From the 15th till the 19th centuries, sand storms moved further east to today's Hebei, Henan and Shandong provinces and the large cities of Beijing and Tianjin.

Formation reasons

Modern studies have found that dusty and sandy weather results from four necessary conditions: powerful and lasting wind, dry climate, loose earth and sparse vegetation.

Chinese meteorologists have drawn an outline of the wind features in China. The country's northeast, north and northwest including the Qinghai-Tibet Plateau have 10 to 75 days of powerful wind, which blows at an average speed of 2 to 4 metres per second.

In China's southwest, south and the Yangtze River drainage area, the wind is rather moderate, at 1 to 3 metres per second. In this area, only five to 25 days have strong wind annually.

Spring and winter are the two seasons when the wind is at its strongest in China. In some areas of North China, the wind can reach 6 metres per second.

Contemporary meteorological studies coincide nicely with the climate records of Chinese history, Wang said. Among the 254 sand storms recorded in annals, 115, or 45 per cent took place accompanied by strong wind.

The places where windy and sandy days occurred most frequently are usually places with dry climate zones, with sparse vegetation growing on sandy earth. The Loess Plateau at the middle and upper reaches of the Yellow River, for instance, is piled high with very loose yellow dust, which is prone to flying into the air even if the wind is not very vigorous.

Zhang also noted that the appearance of sand storms has certain connections with human activities.

In the Qin (BC 221-BC 206) and Western Han dynasties, the rulers moved people to the northwest and cultivated farm fields. This first round of land exploitation in Chinese history brought the first record of dusty days during the reign of Emperor Chengdi (BC 32-BC 7) of the Western Han Dynasty.

In the Eastern Han and following dynasties, the governments could not spare much time from the wars to continue cultivating the northwest. Farmers moved out of the Loess Plateau and the vegetation quickly recovered. It wasn't until 280 years later that the second record of dusty days appeared in the early years of the Eastern Jin Dynasty (317-420).

Before the Ming Dynasty began in the 14th century, the nation's agricultural development mainly concentrated in the upper and middle reaches of the Yellow River, a region where the sandy weather frequented in this period.

The Ming Dynasty saw the unprecedented expansion of cultivation. The vast areas surrounding Beijing, Hebei and Shandong were gradually shrouded in more dusty storms.

However, Wang noted that the undulation of dusty and sandy weather occurrences doesn't necessarily synchronize with the changes of human activities.

From the end of the Eastern Han Dynasty till the early Sui Dynasty (581-618), China was trapped in consistent and numerous wars. The farming population sharply decreased. On the Loess Plateau, nomadic tribes from North China turned the old farm fields into pastures.

Instead of declining or remaining on a low level, the dusty and sandy days of this period rose much higher than previous periods. In the half century between AD 501 and AD 550 during the Southern and Northern Dynasties (420-581), six sandy storms took place, the highest record only to be surpassed until 500 years later, when 13 sand storms occurred between 1051 and 1100.

When the Ming Dynasty reached its prime from 1368 till 1463, the population quickly multiplied, with farm fields expanding at an unprecedented rate. But no sandy storms occurred for nearly a century.

"Human activity has certain influences on the occurrence of dusty and sandy weather," Wang said. "But the determining factor is still with nature itself."

"To prevent or lower the level of dusty and sandy weather, we should protect and restore the vegetation in areas where such natural disasters are prone to happen," Wang said.