Thick fog (VIS 200M @ 09Z) at Heathrow has been a standard part of the
Royal Meteorological Society's 'Weather Log' for many years.

Using these data since 1980, it seems that the early 1980's regularly
experienced Decembers meeting this criterion (i.e. vis at 09Z 200M),
once each in 1980, 1981 & 1982, and twice in 1984.

There was then a marked change with either no such reports, or very few:
for example in the period 1985 to 2005 (21 years), only the years 1990 &
1991 had such events (1 each).

This time, there has been one report with vis 200M at 09Z, that on the
20th (150M); although FG was reported on the 19th and 21st, the
visibility was between 200 & 900M (incl.): so this will be the first
December since 1991 with the visibility at Heathrow 200M at the 0900Z
climatological hour. (see below that month)

This doesn't provide anything like the full story of course as the
particular problem at the moment is a combination of the duration of
persistent fog and the high traffic volume at this time of year.

[1991: Dec]

In 1991 (December), there was a notable spell of quiet, anticyclonic
weather from the 7th to the 15th, with severe frost and dense, freezing
fog reported, though the 'focus' of the latter, as regards persistence,
was across Yorkshire and parts of the Midlands. Of interest to us here
in Bracknell, the temperature early on the 12th fell to -10.8°C: we've
certainly had nothing like that this time around.

From my notes there was also a major air pollution episode on the
13th/14th, and on Christmas Day, the air pressure was 'close to' 1046
mbar over the Channel and nearby areas of southern England.

Martin.


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