Why wet spells can cause lots of fog, and myths

“… AND with all the moisture about after the recent heavy rain, mist and fog patches will probably form later in the night.”

How many times have you heard statements like that in television and radio weather forecasts? It was certainly uttered by at least one presenter as large parts of central and eastern England were affected by fog.

There is a myth in some forecasting offices around the country that a wet season, such as the one we are experiencing, inevitably leads to a higher incidence of fog – when other factors are favourable – than is the case during a dry one. The argument is that excess water lying in the fields is somehow picked up by the air above and then transformed into fog. Broadly speaking, fog formation does not work like that.

However, there is one circumstance where excess surface moisture does contribute: advection or thaw fog. In Britain this typically happens after a lengthy cold spell, when a chill east wind is replaced by a warmer but very moist southerly one. The cold ground, often waterlogged or covered with melting snow, cools the air flowing across it, and because cool air cannot hold as much water vapour as warm air, some of the moisture condenses in the form of water droplets i.e. fog.

Even then the fog formation is a consequence of the cooling effect of surface water rather than a transfer of moisture from the ground to the air.

It can even be argued that the fog that forms on calm, cloudless nights – meteorologists call this “radiation fog” – is less likely to occur when the ground is saturated, especially after several days of very mild weather. Water has a much higher thermal capacity than solid materials such as soil and asphalt, so at night it cools down more slowly.

This means that air in contact with the Earth’s surface will also cool down more slowly over waterlogged ground than it does over dry ground. The slower it cools, the longer it takes to reach saturation point, and the later the fog will form.

It is certainly true that there were several foggy dawns during August 1976 – the culmination of the great drought – whereas fog was not particularly widespread or persistent during the quiet spell in mid-December 2000 following that autumn’s deluge.