Study Notes on Fog

Fog is a cloud in contact with the ground which reduces visibility to less than 1 km. Fog is formed when water vapour condenses on condensation nuclei (or particles) in the air near the ground. When conditions are right these water particles continue to attract more water vapour and grow until the particles become visible. There are however, several conditions that initiate the process of fog formation. Fog usually develops when relative humidity is near 100% and when the air temperature and dew point temperature are close to one another or less than 4°F (2.5 °C). As a result, the water vapour condenses to form water droplets and fog.

Types of Fog:

a) Radiation Fog

Radiation fog forms when moist air is cooled below its dew-point by contact with a cold land surface that is losing heat by radiation. 

The ideal conditions are:

• High relative humidity at low levels so that overnight cooling will be sufficient for the air temperature to fall to below its dew point temperature resulting in condensation occurring.

• Cloudless, or near cloudless skies, to allow a large heat loss at the ground, and subsequent cooling of the air in contact with the ground.

• Light winds to promote mixing of this cooled air through a few hundred feet of the surface (a calm wind tends to restrict the fog to low-lying pockets).

b) Advection Fog

Advection fog develops when warm moist air moves over a cooler surface resulting in the cooling of the air to below its dew point temperature, and subsequent saturation and condensation. Radiation processes frequently assist in the formation and maintenance of this type of fog, but it is still usually called an advection fog. A certain amount of turbulence is needed for proper development of advection fog, thus wind between 6-15 knots are commonly associated with advection fog. Notonly does the turbulence facilitate cooling through a thicker layer, but it also carries the fog to greater heights. Unlike radiation fogs, advection fogs are often thick and persistent.

c) Sea Fog

Sea fogs are usually advection fogs. They develop when moist air that has been lying over a warm water surface moves over a colder water surface, resulting in the cooling of this air to below its dew point temperature.

d) Steam Fog

Steam fog is caused by evaporation from water into overlying colder air, causes the air to become saturated and condensation to occur. The convection currents above the water give rise to the steaming appearance. The fog may remain in situ but any light wind may advect it many kilometres. This is a common occurrence in areaswhere large, shallow and warm waterways and dams exist. In coastal areas where cool land breezes have opportunity to flow across warm seas, steam fog can be extensive and lift into low stratus.

e) Frontal Fog

Frontal fog occurs at the boundary of two air masses rather than within a single air mass. It mainly develops due to precipitation falling from relatively warm air above a frontal surface, evaporating into drier and cooler air below, causing this air to saturate and condense. Such fogs usually form rapidly and are very extensive