Gratuitous pretty picture
15th February 2008
Above: Ice on Loch Laggan before sunset on Friday. Note the high cloud.
We’ve had quite a few days of hard frost down at valley level (the loch is at approx. 250m above sea level) but have had high air temperatures near summits, so classic temperature inversions. Even with these high temperatures (+ 9 or 10 degrees C. at 1000m on some days) the snowpack has refrozen at higher altitudes. The very clear skies have allowed heat from the snowpack (yes, heat) to escape to the atmosphere by long wave radiation, even though the air temperature has been quite high. So the surface layers of the snow have cooled to give hard or very firm underfoot conditions. During the day any snow that is directly warmed by the sun gets a little mushy while the shaded slopes stay hard despite high air temperatures.
The wierdest example of this aspect of atmospheric physics I experienced was seeing heat haze shimmering up from the snowpack with snow drifting around my feet and an air temperature of +7 degrees C. (Cairngorm in March 1998). Convection, conduction, radiation: heat and snow make quite a conundrum.
Well, with more cloud forecast (but staying dry and cool) those South facing slopes are likely to be a little firmer during the day.
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