Based on the conventional observation data, the ground - based 12 - channel radiometer and wind p rofile radar data from October 25 to 27 2007, the fogweather event occurred in Beijingwas analyzed. The results are as follows: (1) The fog event occurred under the large - scale weather background, and the upper, low and surface weather situation were helpful to the fog’s forming and lasting; (2) With the radiometer retrieved p roduction, the upper, low and surface humidity scheme and the fog lastingmechanism could be found very clearly; (3) The surface temperature, the inversion layer thickness and its biggest intensity variation under 2 000 meters were concerned nearly with the fog change and visibility. The fog beginningwas consistentwith the biggest range of the surface temperature dropp ing, and there was the temperature inversion especially grounding inversion during the fog period. The surface relative humidity was above 83% , and 90% for thick fog, 97% for strong fog. There wasn’t some liquid water in the beginning of the fog, but after one hour the liquid water appeared at the level of 100 meters, and itwas biggest at this level before the weather system coming;(4) The horizontalwind speed was smaller under the level of 600 or 700 meters, and near the level of 600 meters the wind shearwas
obvious during the fog period; (5) It kep t the feeble up and down airflow in the boundary layer during the fog period.