Bernard Burton wrote:


"Rob Brooks" wrote in message
...
Or do we call it snow? A covering of ice needles here in Farsley near
Leeds
Temperature down to -4C already at 6pm in mist/fog. Surprisingly the same
event occured on 19 February here in Leeds and Wakefield areas when
measureable depths were reported.


This effect is due to the freezing of supercooled fog droplets due to the
presence of 'ice nuclei' contaminants in the lower atmosphere. The effect
has been much discussed in various meteorological literature over the
years. In this area (Berkshire) we had some notable events in the late
1970's, and the effect was always confined to built-up areas and the
leeward of same. On at least one occasion at Bracknell, the effect was
marked enough to produce
solar halo phenomena as the ice crystals precipitated out. On another
occasion, I was driving towards Bracknell in the evening, and vertical
refraction pillars could be seen above all lampposts and car headlights
over a small area just to the west of the town, where the untreated
pavements had close to a cm of pptn deposits on them, and those a km or so
to the west had
none. If I recall correctly, the air temp was about -2C. There were
several occasions when the effect was present in Bracknell, but not at
Wokingham. It seems likely that the contaminants are generated by a
practice fairly common in urban areas such as the burning of gas or oil in
domestic appliances. I have never seen the effect in a purely rural
setting, except where this abuts onto, and is down wind of, an urban area.


I remember the halo phenomena in Bracknell in the late 70s or early 80s. I
believe that was an ice fog rather than a fall of ice needles. I recall
seeing ice fogs when I was a child - I'd be watching the fog passing by the
window when, all of a sudden, the water droplets would switch to solid
particles of ice. These particles were not needle-shaped and did not fall
to the ground - they floated.

My one memory of ice needles is the one I've described before that occurred
in December 1962. This happened over a wide area. I saw it start in Bedford
with quite large flakes of needles falling from a thick fog and the blue
sky simultaneously appearing overhead. A couple of hours later, at RAF
Wyton, I saw the pavements there decorated with the same type of flakes and
roughly the same concentration. It was a long time ago but I don't recall
that Wyton abutted on to much of an urban area.


--
Graham P Davis, Bracknell, Berks., UK. E-mail: newsman not newsboy