On 08/02/12 20:17, Col wrote:
Eskimo Will wrote:
wrote in message
...
On Feb 8, 7:25 pm, wrote:
The local forecast was talking at length about freezing rain
this evening, and how unusual it is, so I am assuming that it
is to be of the genuine 'supercooled' varity, rather than
just 'normal' rain making cold surfaces slippery.

So what makes freezing rain fall at temperatures when
one would normaly expect snow?
--
Col

Bolton, Lancashire
160m asl

Snow will fall if the whole layer of air is sub-zero. But if the snow
hits a layer of air above zero it will start to melt. If this above
zero layer of air is deep enough it will turn the snow completely into
rain. If this rain then hits another layer of sub-zero air, for
instance a surface inversion, it will be cooled. If the surface
inversion is too shallow the rain won’t have a chance to drop below
zero, thus your 'normal' rain making cold surfaces slippery. If this
sub-zero layer is deep enough the rain’s temperature will have a
chance to drop below zero. Sometimes it will refreeze and give ice
pellets; sometime you will get proper super-cooled rain.
===============================================

Just to add to that excellent explanation that proper freezing rain
is when supercooled rain drops hit a cold surface and freeze
instantly on impact. Freezing rain is also called thus when ordinary
rain hits a frozen surface and freezes after a few minutes. That is
what is expected tomorrow.

I'm confused now.
So what is expected isn't the supercooled variety that freezes
instantaneously, but a rather severe version of 'normal' rain
falling upon a very cold surface?
I guess the meteorological semantics of it are irrelevant when
you are simply trying to warn of ice!

There are some important differences. "Normal" rain will, as Will says,
freeze on a very cold surface. Eventually, however, the rain will warm
the surface it falls on. This will occur quicker on something small,
like a wire, than it would on a road or pavement. Ice would then stop
forming.

"Proper" freezing rain, being super-cooled, will not warm a surface and
may cool it down. This makes it more dangerous for power cables and such
than "normal" rain.

Whereas "normal" rain can only occur as freezing rain when warm air
moves over very cold ground, i.e. on a warm front or similar, the
super-cooled variety can occur on a cold front where the cold air is
likely to be flowing over a warm surface. The rain will rapidly cool
small objects and freeze on them but may take some time to cool
pavements and roads to freezing point. The best example of this that I
experienced was on 30th Dec 1961. In the afternoon, the drizzle that had
been falling all day turned to steady rain. After a while, I noticed
wire fences and telegraph poles had an odd sheen to them. I felt them
and found they were covered with a film of ice. The pavement, however,
was still just wet but, eventually, that too became ice-covered.

--
Graham Davis, Bracknell, Berks. E-mail: change boy to man
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