Dense fog is a severe hindrance to uninterrupted traffic movement. Fog is a common hazard at many airports. It delays the traffic by land or sea. Fogs cause accidents and bring about untold sufferings and economic loss. It is through their effect on visibility that fogs become the most dreaded environmental hazard.
Artificial fog dissipation was used during the Second World War Fog dispersal methods that are economically feasible are of two basic types: (a) Evaporation of fog particles by heating the foggy air or by injecting a drying agent into it. (b) Physical removal of the fog droplets by their precipitation.
The British made a successful use of heating methods for the dispersal of fogs in order to clear their airports during World War II. What will be the amount of heat required to dissipate fog depends on the latent heat needed to evaporate the water in a unit volume of fog and the volume of foggy air to be cleared in a given time.
For this purpose certain special type of burners were used. But the costs of installation of the equipment as well as its operating costs were uneconomically high. But the heat of jet-engine exhausts proved more economical. These methods were suitable for water fogs at temperatures above -39°C.
Seeding of fog with dry ice, silver iodide, carbon black, water or calcium chloride spray has yielded most promising results in the dispersal of super-cooled fogs at airports. Seeding with dry ice or silver iodide from an aeroplane was found useful on fogs in the temperature range -5° to -25°C.
But fogs seldom occur in this temperature range. By applying this method the particles are converted into ice-particles which grow by the process of sublimation and ultimately precipitate. Unfortunately, most of the fogs are of the warm type and are harder to combat.
In fact, warm fog can be cleared only by mechanical mixing of the fog with drier and warmer air from above. In case of shallow fogs, helicopters are made use of in fog dispersal.