Abstract: Using surface observation data from Kunming Changshui International Airport and ERA5 reanalysis datasets, this study analyzes a heavy fog event that lasted 19 hours from January 3 to 4, 2013 and disrupted airport flight operations. This study clarifies the formation mechanisms of the fog from the aspects of circulation patterns and evolutionary characteristics of thermal and water vapor fields. Based on evolutionary characteristics and influencing factors in different periods, the process can be classified into four stages: formation, development, maturity, and dissipation. The primary favorable conditions for fog formation include sufficient water vapor advection, local convergence, the southward shift of the Kunming quasi-stationary front, and low underlying surface temperature. The north-south oriented mountains on the eastern and western sides of Kunming Changshui International Airport, continuous water vapor supplement ahead of the southern branch trough, and the combined warming and moistening effects of low clouds are key factors leading to the prolonged maintenance of the heavy fog.