Characteristics of hazy days in the region of Beijing,Tianjin,and Hebei

Meteorological data(1980~2008) were collected from 107 ground stations in the BTH(Beijing,Tianjin,and Hebei) area.Monthly and Annual numbers of hazy day for each station were calculated using meteorological element and weather phenomena.The annual average hazy days for Beijing,Tianjin,and Hebei depicted similar fluctuations and same trends of increasing.Average number of hazy days at non-urban stations depicted an increasing trend,and the difference in the number of hazy days for urban and non-urban stations had been narrowing over time.Monthly variation of hazy days for the BTH area displayed bimodal trends,with more hazy days in the summer and winter.According to the spatial distribution,the areas with more hazy days were mainly located in Beijing,Tianjin,Baoding,Shijiazhuang,Handan,and Xingtai.At most of stations in the BTH area,average wind speed at 14 o'clock in hazy days was over 1.0m/s lower than non-hazy days,and average relative humidity at 14 o'clock in hazy days was over 20% higher than non-hazy days.