Why Is the Coast Freezing While Inland Is Sunny? A Sea Fog Came Ashore
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A cold wet fog rolls off the North Sea onto the east coast of Britain on bright spring days, dropping the temperature sharply while a few miles inland stays warm.
How it forms
The sea warms and cools far more slowly than the land, so in spring and early summer the North Sea is still near its winter temperature while the air over the continent and over Britain has warmed considerably. When that warm moist air moves across the cold sea surface, the layer in contact with the water is chilled below the point at which it can hold its moisture, and the moisture condenses into a dense fog sitting on the sea. That is advection fog, formed by air moving over a cold surface rather than by the ground cooling at night, which is why it can persist all day.
How it reaches the land
Arrival depends on wind and the effects are abrupt:
- •An easterly wind pushes the fog bank onto the coast
- •Temperature falls by ten degrees or more within minutes
- •Visibility collapses, sometimes to a few tens of metres
- •The fog can stop at a line and go no further inland
- •A few miles west of that line the day is clear and warm
- •It can sit offshore for days and come in each afternoon
Why it stops where it does
The sharp inland limit is what makes the phenomenon so striking and it has a simple explanation. Fog moving over warming land is heated from below, which eventually raises the air above its condensation point and the droplets evaporate, so the bank dissolves at whatever distance that takes. How far it gets depends on how strong the wind is, how warm the land has become and how deep the fog layer is, so the limit moves through the day and between days. Rising ground helps it dissipate faster. On the coast of eastern Scotland and northern England the boundary frequently runs within a few miles of the shore and is visible from higher ground as a wall.
The names it goes by
The same phenomenon carries different names along different stretches of coast and the vocabulary is worth having. The term used here belongs to eastern Scotland and north east England. Further south along the Yorkshire and Lincolnshire coast it is a fret, or a sea fret. The Northumberland coast uses both. Comparable fogs on other coasts have their own names, with the persistent summer fog of coastal California produced by the same mechanism over a cold current, and the fogs of Newfoundland and of Namibia likewise. In each case warm air meets a cold sea surface, and the local name records how familiar the effect is to people living there.
What it does to people
The effects are economic as much as meteorological along the affected coasts. Resorts on the east coast lose trade on days that are warm and sunny everywhere else, which is a long-standing complaint. Airports close or divert, and the North Sea oil and gas industry loses helicopter flying days, which is expensive. Agriculture is affected, since the cool damp conditions slow growth and encourage disease on crops near the coast. Against that, the same fog delivers moisture to coastal vegetation and to certain habitats, and several plant communities on eastern cliffs depend on the humidity it supplies during otherwise dry spells.
The takeaway
Warm moist air moving over a sea still near its winter temperature is chilled from below until its moisture condenses, producing fog that persists through the day rather than burning off like the overnight kind. An easterly wind brings it ashore, dropping the temperature ten degrees in minutes. It dissolves once warming land heats it from below, which is why the boundary can be a few miles inland and visible as a wall.