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Define Fog

A suspension of either water droplets, ice crystals or both at the surface of the earth.


What condition must be occurring, and what are the 2 ways condensation can occur for fog to form

An inversion must be present
the air must be cooled to it's dew point
water vapour must be added to the air to bring the dewpoint up to the temperature


3 factors that control the evolution of fog

Cooling of the air
addition of water vapour


What are the types of fog caused by cooling of the air

upslope fog is formed as a result of adiabatic cooling
radiation fog is formed as a result of radiation cooling
advection fog is formed as a result of advection cooling


what are the types of fog caused by adding water vapour to the air

Frontal fog (warm rain fog)
Steam fog, arctic steam fog
ice fog


radiation fog

-earth continues to re-radiate suns energy after sun sets, cooling the earth
-cool earth cools warmer air above it
-temperature decreases to dewpoint, saturation occurs
-relatively warm air over cold land surface
-forms on land on clear days, usually in the morning
-clear night
-light winds (3-5kts)
-high RH
-condensation nuclei


Advection fog

-Moist warm air moves over a relatively cold surface
-moderate wind speed (13-17 kts)
-high RH
-Condensation nuclei
-can form on land or water
-will not dissipate over water during the day
-if wind speeds are higher, clouds will form at the top of the turbulent layer, lower wind will mean a thinner layer of fog


Upslope fog

-air is lifted adiabatically and cools by expansion to dew point
-moderate wind speed (higher speed will cause cloud)
-high RH
-condensation nuclei


Pre-frontal fog

-warm rain falls through cold nearly saturated air
-evaporation takes place, supersaturation occurs, extra water vapour condenses.
-forms ahead of a warm front and in cold air after cold frontal passage
-warm air over cold air
-warm frontal precipitation
-high RH
-Condensation nuclei


Steam fog

-water vapour is released from warm water to cooler air above
-condensation occurs when dewpoint is raised to temperature
-shallow unstable layer of air above water surface
-happens in arctic when air moves from an ice covered surface to warmer water.


Ice Fog

-Man made
-very cold temps
-strong surface inversion
-formed from ice crystals caused by the addition of water vapour through fuel combustion
-will build up if winds are light
- -30c


2 factors that control the dissipation of fog

-heating of the air
-decrease in water vapour


4 ways of heating the air
(fog dissipation)

-solar radiation
-advection over a warm surface
-adiabatic heating
-turbulent mixing


2 ways of decreasing water vapour
(fog dissipation)

-turbulent mixing of fog with drier air above
-condensation of water vapour in the form of dew or hoar frost