Abstract: Based on the hourly data from Yinchuan weather station,Ningxia Province,China during 1981 to 2013,and according to the national standards and local realities,the level of fog and haze is classified. The differences of the factors that effect the atmospheric diffusion,including the still wind frequency,the thickness and strength of inversion and the mixed height,are analyzed in this paper. The results are shown as followed:Fog mainly appears from the evening to 11:00 of the second day,it reaches its peak at 7:00 and 9:00 in the morning, the fog days in Autumn and Winter are significantly more than that in Spring and Summer;In terms of the strength,the mist fog days accounts for 92% of the whole year,the days of moderate fog,dense fog,thick fog and heavy fog account for 6.8%,6.3%,4.4%,2.3%,respectively. Since the beginning of 21^st century,the number of fog days has obviously increased than that in the 21^th century. Haze usually occurs during 08:00 to 14:00,it mostly appears around 11:00 and is less during other periods of a day. Haze days in winter accounts for 51.7%,followed by autumn,accounting for 26.8%;Mild haze,moderate haze and severe haze accounts for 72%,23.7%, 4.3%,respectively. The number of haze days shows an obvious downward trend during 1981 to 1995. It shows an obvious upward trend after 1996,and reaches its peak in 2013. Dense fog,strong fog and heavy haze occur mostly in autumn and winter. If there is thick fog or heavy fog at night,the mixed layer height will be less than 150 m by 100%. When the haze appears,the potential pollution level is mainly at level 2,the mixed layer height will be between 150 and 350 m by 75%. From the evening to the second morning in autumn and winter,under the condition of still wind,when the air relative humidity is higher than 80%,there will be a fog day by 50% to 80%. However, if the air relative humidity is below 80%,the occurrence probability of haze will be in 40% to 75%,under the condition of small wind,the occurrence probability of mild haze and moderate haze is larger than that under the condition of still wind,and under the conditions of static wind,the occurrence probability of severe haze is larger than that under the condition of small wind. In the fog and haze days,the atmospheric stability is mainly dominated by neutral (class D),and followed by a more stable type (class E).

Key words: Yinchuan city, fog and haze days, meteorological conditions, conceptual model