"Martin Rowley" wrote:

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)

I've just been researching some background on the White
Christmas of 1956 for the DT tomorrow.

An abnormally mild and very disturbed first half of December
gave way to a large anticyclone from the 17th-22nd with
widespread, dense, persistent freezing fog. Sound familiar?

The MWR published visibility data for the intermediate synoptic
hours (i.e. 03, 09, 15 and 21). Data for Heathrow showed 21
such obs within fog limits, of which 11 were below 200m.

Croydon had 17 with 7 below 200m, and 1 of those below 40m.

We may guess that all or most of these occurred during that
single foggy episode. The commentary notes: "Fog ... persisted
throughout the day over much of England from the 19th-23rd."

In 1956 the anticyclone migrated to Scandinavia just before
Christmas, and vigorous depressions on the Atlantic produced
a southeasterly 'squeeze' across the UK resulting in a prolonged
snowfall during the 25th/26th. The mild air won though and many
places recorded no other significant snowfall during the entire
winter. Winter 1956-57 was the warmest between 1950 and
1973 inc.

Philip