There is an old Irish rule of thumb which tells us: Ceo soineanta ar abhainn, ceo doineanta ar cnoc - "river fog, fine weather fog, but hill fog augurs ill". Be that as it may, there is one location where fog is always bad. Fog at an airport disrupts aviation schedules.
Over the years, two distinct techniques have been used for getting rid of runway fog. Precipitation methods require the water droplets in the fog to grow in size so that, eventually, they fall to ground and leave the air transparent.
The vaporisation method, on the other hand, depends on the fact that if an area of fog is heated sufficiently, moisture will be re-absorbed into the atmosphere from which it was condensed originally.
One way to encourage precipitation is to spray the runway area with electrostatically charged water droplets. The assumption is that these charged droplets attract other droplets of the opposite charge within the fog. The drops grow bigger and then fall to the ground.
Spraying with hygroscopic particles, such as common salt or calcium chloride, may also sometimes be effective. If a bank of fog is "seeded" with such particles, they absorb water vapour from the saturated air and grow large enough to fall like raindrops.
By removing the moisture in this way, a point is ultimately reached where the air is no longer saturated, and a temporary clearance comes about. Unfortunately, in practice any clearance tends to be ephemeral. In addition, it is difficult to ensure that the "hole" in the fog occurs precisely where you want it - on the runway.
Fog dispersal by vaporisation requires gas or petrol burners to be installed along the runway. During and shortly after the second World War, the British had great hopes for FIDO - the Fog Investigation and Dispersal Operation. FIDO worked to some extent, but because great amounts of heat were required to produce the desired effect, vast quantities of fuel were needed and the practice became prohibitively expensive. So FIDO died.
In the 1960s the French aviation authorities resurrected the idea, using old jet engines strategically located to provide the necessary heat. They called it Turboclair, and it was credited with some success at Orly and at Charles de Gaulle airports.
None of these techniques has achieved widespread popularity. Nowadays, efforts to cope with runway fog tend to be concentrated on the development of more sophisticated electronic landing systems - the idea being to allow aircraft to operate with electronic eyes and guidance systems so that it does not matter whether there is fog around or not.