Fog simulation and prediction are becoming increasingly important in China because of the great impact of fog on traffic and other human activities. More studies are needed to have a better understanding of the formation mechanisms and life cycles of fogs. This work uses data from two fog cases observed in Wuqing, Tianjin, in 2009. The data include aerosol size distribution, fog droplet size distribution, fog liquid water content, and meteorological properties. The results show that increasing aerosols can increase the number concentration of fog droplets and decrease fog droplet size, which is consistent with the first aerosol indirect effect found in clouds. It is also shown that increased aerosols can lead to lower visibility in fogs. This work demonstrates that the first aerosol indirect effect plays an important role in fogs.