Temporal Variation and Cause Analysis of Fog Days in Xi'an City during Last 50 Years

Based on the fog days data and other basic climate data from three stations of Xi′an,Jingyang and Chang′an,the variations of fog days was studied.By comparing the temporal variation of three stations,especially the long-term variation,the difference of fog days variation between city and its surrounding areas was discovered.The result shows that the inter-decadal variations of fog days changes notably after 1980s.In detail,the fog days of Jingyang and Chang′an both increase obviously,especially in autumn and winter.However,the fog days at Xi′an decrease obviously in all four seasons.The variations of precipitation and temperature are the reasons for the variation of fog days except relative humidity.In addition,the sharp decrease of fog days is affected by the increase of nighttime temperature after 1990.