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Sea
Fret
by Gail Pirie |
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 Fret,
haar and fog - what's the difference? BBC Scotland Broadcast
Meteorologist, Gail
Pirie, explains.
What are they?
Quite simply, all of the above are one and the same. They are
examples of a weather phenomenon more commonly known as ‘sea
fog’. And no matter what you call it, it is the damp, drizzly
stuff that can ruin what would otherwise be a fine day.
The variation in name simply arises from the locale in which
you happen to find yourself when the fog rolls in off the sea.
On the east coast of Scotland sea fog is known locally as haar
or North Sea Haar, and it is often said to plague local residents
during the summer time. Likewise, its English counterpart, fret’,
or ‘sea fret’ can make summer days on the east coast of England
miserable.
So why does this weather phenomenon spoil a day that could otherwise
be spent on the beach?
Where does sea fog come from and how does it form?
Sea fog usually occurs between April and September. But is most
likely on the east coast of the UK, or over the Northern Isles
during early summer, before the sea has started to warm up.
It forms when a parcel of warm air passes over the notoriously
cold North Sea.
The warm air at the bottom of the parcel is cooled by the cold
air below, until it can no longer ‘hold’ the moisture that was
previously contained within. Therefore, it releases some of
the moisture in the form of liquid water through condensation. Add an onshore component like a wind of 5-20 mph, and the cooling
in the bottom of the warm parcel of air is spread upwards and
generates a fog; sea fog. Moreover, if the wind is coming from
a direction between north and south east, the sea fog will make
its journey from the sea over the land.
A number of factors determine the locational extent of sea fog
and whether it will disperse quickly or linger throughout the
day. For instance, if the land is warm when the fog rolls in
from the sea, it will readily disperse. This is due to a smaller
temperature difference between the surface of the earth and the
bottom of the parcel of air.
During the night, when temperatures on land drop, the
sea fog can penetrate a long way inland and linger till the
next morning. Should there be a blanket of fog greeting you
in the morning, then sunshine is needed to burn it back to the
coast. But if the sea fog is very thick, sunshine is unlikely
to burn through it. And if the wind is blowing steadily in from
the sea, any 'haar' that is dispersed will be quickly replaced with a fresh batch.
So all in all, sea fog is usually an unwelcome guest. The length
of its stay is determined by wind speed and direction, sea temperatures
and humidity levels in the bottom few thousand feet of the atmosphere.
All of which makes forecasting its dispersal complex.
Related link
Weather Basics - Mist and Fog
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