The Different Characteristics of Sunny Visibility over Southwest China in Recent 50 Years

A data set of horizontal visibility recorded at 93 stations in Southwest China was used to analyze. The result showed that sunny visibility in Sichuan Basin was lower than other places. The sunny visibility showed a remarkable decline over the past 50 years in big-medium cities, but small cities showed an opposite trend. The trends of sunny visibility in big-medium cities showed obvious decadal changes in over four provinces, revealing a slight increased in Chongqing and Sichuan after the year 2000. However, Guizhou and Yunnan still kept a downward trend. Trends of scattered radiation and relative humidity in big-medium cities showed an opposite change characteristics, the former was upward and the latter was downward. Higher aerosol index in Southwest China still appear in Sichuan Basin, which can rise up to 5.6, having a similar characteristic with sunny visibility. Recent trend of aerosol index still keep upward change, but sunny visibility has an upward change after 1992, which can be attributed to the improved quality of atmospheric environment.