8. Precipitation and Fog

Although generally not discussed in the literature dedicated to fog, the presence of precipitation can have significant influences on the presence and evolution of fog. In this chapter, influences of precipitation are described, with a particular interest in its dual role in fog formation and dissipation. By the same token, the chapter also discusses a relatively overlooked phenomenon: fog formation in precipitation areas within mid-latitude cyclonic systems. A description of the multi-scale mechanisms and interactions influencing the presence of fog under this scenario is provided through the analysis of historical weather observations, theoretical concepts and results from detailed numerical modeling of rainfall evaporation. This updated description shows that precipitation-related fog formation and dissipation is determined by intricate multi-scale interactions, involving specific conditions over a wide range of scales. These include synoptic scale circulations leading to the formation of temperature inversions to the microstructure of precipitating hydrometeors. The latter determines the lagged temperature adjustment of the precipitating particles and hence their evaporation rates in near or saturated environments (influencing fog formation), as well as their size-dependent capacity to scavenge fog droplets (influencing fog dissipation).