Analysis of Characteristics and Trends of Atmospheric Visibility in Shizuishan City of Ningxia

Based on visibility,relative humidity,weather phenomenon and related other data in the Shizuishan of Ningxia during 1980—2009,using two statistical methods: cumulative percentage and frequency ofvery good visibility,characteristics of visibility caused by only air pollution were studied.The results showed that the visibility has declined in Shizuishan in the past 30 years.The most marked decline of visibility was in summer,but the decreasing trend of visibility was improved in recent years.The frequencies of very good visibility decreased obviously and the largest range of visibility was declining in summer after 2000,the decreasing range about visibility and the frequencies of very good visibility were relatively large.