Consecutive extreme visibility events in China during 1960–2009

Features of consecutive extreme visibility events, such as the annual maximum duration of consecutive good visibility (≥20 km) and bad visibility (<5 km) and the annual frequency of consecutive extreme visibility events (≥3 day), and their visibility values were explored over China in recent 50-year period. Different features of consecutive extreme visibility events in large, medium and small cities were compared. According to the spatial characteristics of consecutive extreme visibility events, China can be divided into two main regions: one region extends from the north part of northern China and westwards to northwestern China and the western part of southwestern China, where consecutive good visibility events have much higher frequency than do other regions, which include the southern parts of northeastern and northern China, the middle and lower reaches of the Yangtze River and southern China. The degradation of visibility in consecutive extreme visibility events in the northern part of China is less than that in the southern part, and an increased visibility value during these events was found in the northern part of northern China. The frequency of consecutive good visibility events decreased in all cities especially after 1980, and their regression coefficients reached a minimum in small cities and a maximum in large cities.