The fogs of autumn

  • Foggy pylons. Photo: Brendan Jones

    Walkers peer down onto fog in the South Downs of Hampshire. Photo: Brendan Jones

    Fog blankets Surrey in 2006. Photo: Brendan Jones

  • The fogs of autumn
    17.11.2011 16:03


    “Much fog in autumn, much snow in winter”.

    The excitable chatter in some portions of the media recently would have us believe that a cold winter is on the way, yet again. Are there any ways of determining whether this is true? If the opening quote of folklore is to be believed, then weather conditions in the next few days may be food for thought. As the daylight shortens and the temperature drops, very few weather phenomena evoke images of autumn’s advance, as a blanket of thick, featureless fog can.

    Large swathes of north-west
    Europe have been plagued by fog over the last few days, with visibilities as low as 30 metres in some areas. Fog can, of course, form all over the world at any time of year, but it is most common across Europe in the autumn months. There are various species of fog, all forming in their own way, but all require two key ingredients; temperature and moisture.

    Water, in its gaseous form, is called water vapour. It is suspended in the air all around us, all of the time. The warmer the air, the more water vapour it can suspend. So what happens to the water vapour already held in the air, as temperatures drop sharply on autumnal evenings? Well, in theory if the temperature of air reduces then the amount of water vapour it can hold should also reduce. This results in there being an excess amount of water vapour in the air, a state known as saturation (or 100% relative humidity).

    Once air becomes saturated, water vapour must be lost from the ever-cooling air, and this produces condensation. We see condensation on a daily basis as dew, coating grass, cars and windows in water. However, water vapour can also condense out into the air as well, to form small, hovering water droplets. We know this as fog. The more water vapour that condenses out to form fog, the thicker the fog becomes and the poorer our ability to see through it.

    Provided that winds are light, fog can form and remain in place for many days. As this weekend approaches, settled autumnal conditions are threatening to promote copious amounts of fog on Saturday night and Sunday night, particularly across England and Wales. Visibility may well drop to less than 100 metres in places which can obviously threaten some disruption to transport. Thankfully, as the wind freshens and rain moves in later on Monday, the fog is expected to disperse, at least for now.

    By: Brendan Jones