BEIJING, Mar 3, 2001 -- (Agence France Presse) Beijing was shrouded in a yellow layer Saturday as the fourth sandstorm of the year swept down from the arid plateaus of north--central China covering the area in sand and grit.

Winds measuring between force five and force seven were expected to swirl the yellow grit throughout the day hampering visibility and causing respiratory problems, the Beijing Morning Post said.

Sudden and violent sandstorms in northern China normally strike in spring, but this year three such storms engulfed Beijing in January in a sign they were becoming more frequent, Yang Gensheng, a desert expert at the China Academy of Sciences told Xinhua news agency.

The storms were attributed to repeated cold fronts coming down from the north coupled with expanding deserts throughout northern China that have grown due to economic activities, like logging, strip mining, and the blind expansion of crop lands, he said.

According to Wang Tao, a researcher at the China Northern Desertification and Administration Research Project, only five major sandstorms struck northern China in the 1950s, but this number grew steadily to over 20 in the 1990s.

However, some experts argue the storms are not increasing in frequency and that record snow fall this year in Inner Mongolia and in the Republic of Mongolia would reduce the amount of dust stirred up.

Last year Beijing was struck repeatedly by the storms in one of the worst years on record.

Besides regularly engulfing the capital and the northeastern port city of Tianjin, the sandstorms are now spreading as far south as the Yangtze River and Shanghai, Wang told Xinhua.

"Sudden and violent sandstorms are witnessing an ongoing trend of increasing frequency, shorter periods in between storms and increasing strength," he said.

((c) 2001 Agence France Presse)