Fog
- cloud on the ground; surface based cloud composed of either water droplets or ice crystals
- forms by
1) cooling air to its dew point, or
2) adding moisture to air near the ground
Types of Fog (classified by formation)
1) radiation - forms when terrestrial radiation cools the ground which in turn cools the air in
contact with it to the dew point temperature; occurs with clear skies, little or no wind,
small T-DP spread; is relatively shallow & forms at night & daybreak; winds > 5 kts
disperse it (ex: ground fog; San Joaquin valley in winter)
2) advection - forms when moist air moves over cold ground or water; is more extensive &
persistent; winds > 15 kts disperse it (ex: U.S. west coast; San Joaquin valley in
summer)
3) upslope or orographic - forms when moist, stable air is cooled adiabatically as it moves up
sloping terrain; is quite dense & extends to high altitudes; dissipates when wind ceases
(ex: eastern slope of Rockies during winter & spring)
4) evaporation or precipitation-induced or steam fog - forms when warm rain falls thru cool
air, evaporation from the precipitation saturates the cool air & forms fog; can be quite
dense, extensive & persistent; commonly associated with warm fronts; same mechanism
that produces a condensation trail (contrail) behind jet aircraft or moist exhaled breath on
a very cold day or clouds over geysers; when occurs at sea is called sea smoke (water
molecules evaporate from water surface into cold overlying air)
5) ice fog - forms in cold weather with temperature much below -18°C (0°F) & composed of tiny
ice crystals (ex: Arctic regions)
Mist - very thin fog which often precedes & follows fog
Haze - concentration of salt or other dry particles not classified as dust; occurs in stable air &
may extend as high as 15K ft; unlike fog which evaporates, haze must be dispersed