The urban thick fog features and its relationships to traffic weather services in Beijing

Based on the observation data of the visibility below 1000 m from 18 automatic weather stations on expressway surface and traffic data in Beijing area from 2007 to 2010, the urban thick fog features and its relationships to traffic weather services in Beijing was investigated.The main conclu-sions are as follows: (1) more than 90% of expressway closed cases in Beijing owes to the thick fog. (2) The thick fog whose visibility is below 200 m happens mostly between 05 a.m. and 09 a.m. and the less between 12 a.m. and 18 p.m. in a day, while in a year, it happens mostly from September to December and less in June and July. (3) About 80% of the thick fog cases with visibility below 200 m happen in the Daxing and Tongzhou districts in the southeast parts of Beijing, while the few cases happen in urban area.(4) The thick fog has features like suddenness and“trunk-like”fluctuation before the formation of thick fog. (5) During the period of thick fog the visibility changes with the meteorological elements such as wind, temperature and relative humidity. The southerly wind and easterly wind near the ground are advantageous to the formation and development of thick fogs, while the northwest wind is advantageous to the dissipation of thick fogs. It is beneficial to the formation and develop-ment of thick fog that the wind speed becomes low, the relative humidity is increased, temperature drops down, and the visibility is reduced. The wind speed, relative humidity, temperature, and the visibility are all changed a little bit as thick fog maintains. During the period of thick fog dissipation, the wind speed becomes high, the relative humidity is reduced, temperature rises,and the visibility becomes better. But the reduction of relative humidity is later than the rise of the visibility.