"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.

--
Bernard Burton
Wokingham, Berkshire, UK.

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