Fog

Fog is denser so contains more water droplets than mist. The official definition of fog is a visibility of less than 1000 metres. This limit is appropriate for aviation purposes, but for general public and motorists an upper limit of 200 metres is more realistic. Severe disruption to transport occurs when the visibility falls below 50 metres.

Fog is a common problem here in the UK as we have four different types: radiation fog, advection fog, hill fog and coastal fog. All of these can be a hazard for road users and also airports.

Radiation fog
Radiation fog is formed on clear, still nights when the ground loses heat by radiation, and cools. The ground in turn cools the nearby air to saturation point, thus forming fog. Often the fog remains patchy and is confined to low ground, but sometimes it becomes more dense and widespread through the night.

After dawn, fog tends to disperse because it is 'burnt off' by the incoming solar radiation, some of which penetrates the fog and reaches the ground. The ground heats up, as does the layer of air near it. Eventually, the air reaches a temperature where the minute fog droplets evaporate and the visibility improves. However, in winter fogs can be very persistent.

Advection fog
Advection fog is formed when very mild moist air moves over a cold ground. This can often happen in early spring in the UK when mild southwesterly winds moving across the country over snowy or icy ground. The lower layers of the air get cooled down rapidly to below the temperature at which fog forms.

Hill fog
Hill fog or upslope fog, as its name implies, is formed as mild moist air is forced to ascend a hill or mountain range. As the air moves up the windward side of the mountain it cools down, and again if the air becomes saturated then cloud is formed which, if below the top of the hills, gives fog.

Coastal fog
Some coastal regions of the British Isles suffer from 'sea fog' which forms when moist air is cooled to saturation point by travelling over a cooler sea. The wind may then take the fog into coastal regions. This type of fog tends to occur in spring and Summer, and particularly affects south western and North Sea coasts.

Steam fog
'Steam fog' is sometimes seen rising from the ground after a shower. If the ground is warm, the water from the shower may evaporate. If the air above it is saturated and cannot hold any more water, the excess moisture condenses and looks like steam. This 'steaming' can also be seen at the Poles, when cold air sweeps over a slightly warmer sea. The sea warms the air a little and some water evaporates. The warmer damper air then rises, cools again and the moisture condenses.

Freezing fog
Freezing fog is composed of supercooled water droplets (i.e. ones which remain liquid even though the temperature is below freezing-point). One of the characteristics of freezing fog is that rime - composed of feathery ice crystals - is deposited on the windward side of vertical surfaces such as lamp-posts, fence-posts, overhead wires, pylons and transmitting masts.

So, as a driver, when we have still cold nights in winter, spring and autumn be wary of fog forming late in the night especially in valleys. Also if the forecast is for moist southwesterly winds coming in after a cold spell watch out for fog over the hills or another favourite place is on the western sides of hills in the warm sector between the warm weather front and the cold one.