Favoured synoptic environment for St/fog Fog (or low Stratus) is formed, when moist air near ground level starts to condensate. This condensation can be produced in the atmosphere by three mechanisms:
- By ascent and resultant cooling of an air parcel.
- By radiative heat loss.
- By the mixing of two parcels of slightly unsaturated air initially having different temperatures.
The necessary condition for Fog formation is a sufficient moisture content in the lowest layers of the atmosphere. Radiation Fog formation typically calls for clear skies, ample moisture in the surface layer and light winds. Preferred locations to fulfil these conditions can be found adjacent to high pressures over land (mostly during the winter season) with associated weak pressure gradient. Wetness of soil significantly increases the chances of Radiation Fog, for which reason a very favoured situation for Fog formation is the sky clearing and wind decreasing in the evening after a rainy day. Too strong wind will most likely create Stratocumulus than Stratus.
The process of formation, maintenance and disssipation of Fog can be described briefly as follows:
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The formation of Radiation Fog:
- The cooling of the air near the ground.
- Increasing thermal stability, weakening turbulence above ground.
- Excess water vapour condensing into Fog droplets.
- Fog becomes more uniform in the vertical, with a well-defined top.
- The spreading of a cloud sheet over the Fog, reducing the radiative cooling at the Fog top layer.
- Heat from the ground lifting the Fog into low cloud (Stratus), improving visibility.
- Increasing wind shear at Fog top caused by increasing wind above the Fog may increase the entrainment of dry air into the cloud and accelerate the rate of dissipation.
- In mature Fog the settling of droplets to the ground or onto vegetation contributes significantly to the dissipation.
- Except in mid-winter at higher latitudes, solar radiation can penetrate to the ground in places where Fog is thinner, heat the ground and evaporate the Fog.
References- Manual of Synoptic Satellite Meteorology
- STULL, R. B. (1988): Boundary Layer Meteorology. Kluwer Academic Publishers
- ROACH, W.T. (1994): Back to basics: Fog: Part 1- Definitions and basic physics. Weather, Vol. 49, p. 411 - 415.
- ROACH, W.T. (1995): Back to basics: Fog: Part 2 - The formation and dissipation of land fog. Weather, Vol. 50, p. 7-11.
The dissipation of Radiation Fog