On 19/01/2013 15:55, Eskimo Will wrote:

"Yokel" wrote in message
...
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?

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

--
- Yokel -

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