The Foggiest Idea

Fog is a weather hazard which can create all sorts of problems and delays.  Different meteorological scenarios produce different types of fog, and understanding these can help us anticipate when it may form.

Reference ACWM Chapter 10 on Visibility

The requirement for the formation of all types of fog is high relative humidity.  Typically this happens as the air temperature is lowered to the dew point (but not always - with steam fog, the dewpoint is raised to the air temperature).  It also involves condensation nuclei and some physical process which causes condensation to occur. 

Types:

• advection fog: cooling of warm air as it moves over a colder surface (land or water)
• radiation fog: cooling of the earth's surface at night (land)
• upslope fog: adiabatic/expansion cooling 
• steam fog: water vapor is added to very cold air
• ice fog: addition of water vapour to air through fuel combustion which then sublimates into ice crystals
• frontal fog: evaporation of rain that falls from the warm air into the cold air preceding a warm front

POP QUIZ!

What is the difference between fog and mist? (TC AIM MET 3.15, WMO Code, Table 4678)