Advection Fog

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Advection fog develops when relatively warm moist air blows over a much cooler land or sea surface. In Britain there are three situations where this commonly happens:

  • In the winter, when a cold snap comes to an end and the ground is still frozen or very cold, and a warm front introduces moist tropical maritime air. Land areas away from the coast are worst affected.
  • In the spring and summer, when warm tropical continental air blows from the continent across the cold North Sea, and to a lesser extent, the English Channel. This often causes extensive sea fog along the coast
  • When local sea breezes develop, again in the spring and summer. All coastal areas are prone to this but especially those facing outwards onto a large area of sea.

As advection fog over land areas is not particularly photogenic and cannot easily be distinguished from thick radiation fog, all the following images are of sea fog photographed in late May.

Sea Fog

Saint Cuthbert's Island, Lindisfarne

With the tide out, and some of the sand flats exposed, the sun has been able to war the air a little, evaporating the fog, especially towards the near shore. Beyond the island, there is an inlet of sea keeping the air cold and preserving the fog.

The tops of the hills on the mainland can just be seen, peeping out above the fog in the top right part of the photograph. A mile or so inland, it was a beautiful sunny day, and further inland it was quite warm.

Around the coast of Lindsifarne the temperature was around 12oC, while several miles inland it was more like 19oC.

Lindisfarne harbour

This photograph was taken an hour or two later when the temperature had risen a degree or two, but not enough to completely evaporate the fog, which can be seen drifting in from the sea towards the fishermen's huts and the ruins of the priory

In late May sea temperatures off the North East coast are often still below 10oC, which accounts for the frequency of fog whenever the wind blows gently from the eastern half of the compass.

Lindisfarne Castle

Taken shortly afterwards, when the temperature had reached its peak for the day, there was enough heat penetrating the fog to warm the surface of the island and its harbour waters.

Consequently the lower regions of the fog have evaporated leaving it clear over the harbour. The castle, however, is closest to the open sea so a raised fog is still drifting in from the east blurring its upper parts

By this time in the afternoon, the temperature had reached the dizzy height of 14oC on the island, but this contrasts with inland parts of Northumberland exceeding 25oC. Situations like this, where the coastal fringe is significantly cooler are common in the later spring and summer, and for those living on the North East coast it can be galling listening to television reports of a heat-wave!

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