In order to study the physical and chemical processes of Nanjing winter fog, reveal the relationship between air pollution and the chemical structure of fog water and establish fog forecasting methods， a comprehensive observation of fog during the winter of 2006 and 2007 was made in NUIST (Nanjing University of Information Science and Technology). Based on the obtained fog data and conventional meteorological observation data during 2006 to 2008 at Nanjing Meteorological Observatory, the synoptic situations and macromicrostructure characteristics of precipitation fog were analyzed. Results show that: The weather situation when precipitation fog happened can be divided into 3 kinds: frontal surface pattern, pattern under high pressure, and low pressure pattern with inverted trough; Invasion of weak cold air is an important condition for the formation of precipitation fog; Another important condition is that a strong inversion layer exists, namely the temperature is higher in upper of the boundary layer than it near the surface; Rain enhancement or increasing surface wind speed, intrusion of the durative dry cold air are likely to result in dissipation of precipitation fog, the nature of which is evaporation fog. Compared with other types of fog, the number density, water content and average diameter of precipitation fog are smaller. Fog droplet spectrum is very narrow, and its curve exponentially rapidly decreases.