"Yokel" wrote in message
...
On 19/01/2013 15:55, Eskimo Will wrote:

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

...

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.

...



"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?


Felt roofs tend to be of fairly solid construction - the felt is normally
tacked onto wooden planking below - so I think you are probably right.
Considering your location, if your shed roof was not of fairly solid
construction I should imagine it would have made its way elsewhere some
time ago.


Thanks, I felted it myself about 8 years ago with 3 layers of felt and it is
solid.

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