Bernard Burton wrote:

This phenomenon was very prevalent in this area of Berkshire in the 1970s
and 80s. I wrote a note on it in the Journal of Meteorology Apr 1976,
p232. It was sometimes very evident when travelling through Bracknell by
train, as the lying 'snow' from the ice crystal deposit was 0.5 to 1 cm
thick in specific areas, with none at all in other locations close by. On
one occasion I collected a sample of the crystal deposit and sent it for
analysis to try to determine the likely nucleation agent, but the analysis
found only standard pollutants. I also requested a list of chemicals being
discharged from industrial premises in Bracknell from the local council,
but was refused this information.
On another occasion the fog was thin enough to show the sun shining fairly
brightly, and a strong 22 deg halo was present as the crystals were
falling. Another occasion at night showed light pillars above all street
lamps and car headlights in fog with an air temp just below zero, and ice
crystal precipitation occurring.


I remember one of these ice fogs in Bracknell around 1980. Temperature was
about -6C all day with sun visible at times through the fog, showing an ice
pillar. There was a lot of rime on the trees and there was enough air
movement to dislodge this as a fine powder - might have acted as nuclei?

From when I was a young child in Northamptonshire, I had noticed that, with
temperatures a little below freezing, the fog would switch from water
droplets to ice particles and back again.

Bracknell used to get some really dense fogs when I was there from 1965-73
which I thought a bit surprising for a smokeless zone. A couple of
occasions I recall the visibility dropping to less than five metres. Since
my return in '78, I don't recall seeing any dense fogs.

--
Graham Davis
Bracknell