Abstract:

 Using pollution data, hourly surface wind, humidity and visibility in Xi′an from 2006 to 2012, the number of haze days calculated by theoretical prediction and observation were compared. Characteristics of haze days and clean days were analyzed, and corresponding meteorological elements were discussed. The results show that frequency of haze days is high in dry season and low in wet season. Effect of the surface wind on the haze weather is significant, and average daily surface wind is below 1.5 m·s^-1 in most haze weather. It is equal or less than 1.0 m·s^-1 in dry season, and even less than 0.5 m·s^-1 in an extreme case. The number of clean days is higher in dry season than in wet season, which is due to the strong wind speed and low humidity in dry season.

Key words: Wind field, Haze weather, Clean weather, Dry or wet season