北京春季风沙的分析

本文分析了1951—1980年北京30年风沙资料,研究得出:(1)、北京风沙春季最多,占51.6％;冬季次之,占30.5％;夏秋最少,均不足10％。(2),北京春季风沙以能见度在1—10公里范围内的扬沙为主,约占2/3;浮尘占1/4多;能见度小于1公里的沙尘暴最少,仅占1/12。(3),北京春季风沙87％的沙尘是来源于本地,有13％来源于干旱沙漠地区和黄土高原。(4),引起北京春季风沙的风速需在4.7米/秒以上。降水能抑制风沙形成。(5),北京春季风沙与造林绿化密切有关。解放前,北京树木和绿地很少,因此1951—1960年期间风沙极为频繁,平均每年春季约30次,约每三天有一次风沙日。解放后,大力开展绿化造林,到1961年,绿地扩大了四倍多,树木数目增长了200多倍,所以1961—1970年期间,春季风沙大幅度减少,平均每10天有一次风沙日。1966年以后10年,绿地减少了11％,树木减少了40％,因此,1971—1980年的最近10年期间,春季风沙又有回升趋势,平均每周有一次风沙天气。北京的绿地面积再扩大两倍,即可基本上制止风沙为害。

ANALYSIS OF SPRING DUST STORM IN BEIJING

Results of analysis of the dust storm data in Beijing area during the pariod of 1951-1980 are as follows.(1) Dust storms were most frequent (51.6%) in Spring;less frequent (30.5%)in Winter;and much less frequent (<10%)in Summer and Autumn.(2) About 2/3 of the Spring dust storm was caused by blowing sand with the visibility within a range of 1-10 km; about 1/4 caused by sand haze; only 1/12 was sand storm,its visibility is less than 1km.(3) 87% of the sand and dust in the Spring dust storms was originated locally,while 13% of that came from dry deserts and loess plateau north of Beijing.(4)The lowest wind speed that would cause a dust storm in Beijing is 4.7 m/s.Rain is effective to subdue a dust storm.(5) Prevalence of the dust storm was also closely related to tree and lawn planting.Before 1949,trees and lawns were rarely planted in Beijing,thus between 1951 and 1960 dust storms were most frequent,with an average of about :30 storms each Spring,about one storm per three days.After liberation,great efforts have been made to develop tree planting and growing lawns.By 1961,the area of lawn has been increased by more than four times and number of trees planted by more than 200 times.As a result,Spring dust storm in Beijing decreased greatly between 1961 and 1970 with an average of one storm per ten days.In the ten years after 1966,area of lawn was decreased by 11% and number of trees by 40%,thus from 1971 to 1980 dust storms have increased again in the last ten years with one storm a week as an average.If green area would be increased two times than as now is,Beijing would be almost freed from sufferiny of Spring dust storms.