Layers of fog

“A reader asked recently about fog occurring in layers,” says Roderick Stewart of Dronley. “Fog forms when humid air is cooled to the point where the moisture condenses into an opaque cloud of water droplets, fog, and this tends to occur in still weather when air can form layers of different temperatures and different humidities.

“The layer of air immediately above water or wet grass will naturally be humid and will also be cooled by the water below, so when the temperature drops at the end of the day it will form the characteristic low-lying layer of mist so often seen ‘boiling’ out of low ground.

“On a clear evening, the layer of air near the ground can cool particularly rapidly as ground heat is radiated into space. However, if it has also been a sunny day, the dark tarmac of a sunheated road can remain warm after the surrounding air has cooled, so the layer nearest the road will remain clear. This is why fog-lights are mounted lower than normal headlights; to take advantage of this fog-free layer.

“During the Second World War it was found that burning petrol down the sides of a runway could warm the air sufficiently to clear fog for landing aircraft and in the days of sailing ships such as HMS Unicorn it was common for lookouts at the top of the masts to be in clear air when the deck below was shrouded in fog.

“Such layers of fog can be remarkably well defined: I remember sailing from Gibraltar one glorious, misty morning when all that could be seen from our bridge of the other ships in our group was a line of masts, bridges and funnels floating mysteriously over an ethereal white mist, the effect rather spoiled by a higher, dirtier layer of funnel smoke!

“Similar temperature layers occur within the oceans, where they act as mirrors to reflect sonar waves allowing submarines to hide within a layer.”