"Yokel" wrote in message
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On 19/01/2013 09:03, Eskimo Will wrote:
0850

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Amazingly still dripping and icicles on shed roof (unheated), there must
have been a lot of heat stored on the surfaces before the snow as we did
not have a frost in this current spell before the snow came.

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"Radiation thaw"?

We had this situation at Ashurst (New Forest) both yesterday and today.
According to the thermometer in my back garden the air temperature has
remained below freezing both these two days, helped by the 9 cm of snow we
had yesterday. But there has been a significant thaw on both days - as I
write water is dripping from the branches of trees as snow on these is
melting.

Whilst the thawing on the roads etc is very likely due to thermal storage
following the early January mild spell, snow is an extremely efficient
absorber as well as radiator at infra-red wavelengths. There is a nice
layer of relatively warm low cloud only about a thousand feet above the
ground and I reckon the downward radiation from this, helped by some
contribution from the diffuse solar radiation as well as ground "storage",
is steadily "cooking" my snow cover.

Today's thaw is only a slow one and I reckon the Ashurst Observatory is
good for at least two more days with "Snow Lying". But when a cold spell
finishes, low cloud in a mild SW breeze will thaw snow quicker than
anything apart from heavy warm rain. It's a lot quicker than direct
sunshine, most of which is reflected and which also has to compete with
the snow radiating energy to that part of the sky (nearly all of it) from
which the sun isn't shining.


Interesting, but I was in cloud this morning. Also other icicles elsewhere
are not dripping. So I don't know but I agree with what you say about cloud
cover. The shed roof is felt so I wonder if that retains heat better than
cast iron gutters where the other icicles are?

Cheers,
Will
--
http://www.lyneside.demon.co.uk/Hayt...antage_Pro.htm
Will Hand (Haytor, Devon, 1017 feet asl)
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