Why has the snow frozen when it’s warm?

24th February 2019

The temperature last night at 900 metres didn’t drop below 3 Celsius but the snowpack which was wet and soft yesterday has frozen hard today at all elevations. Without going into the physics and chemistry of water, the cooling that causes the snow surface to freeze can be compared to the human body cooling by sweating. It isn’t the sweating that cools you directly but mainly the sweat evaporating. The same thing happens when water evaporates from the snowpack. Snow can also turn straight to water vapour missing out the liquid state (sublimation) and this also causes cooling.

Humidity is another important factor in this process. The drier the air, the more evaporation and sublimation that happen and the greater the cooling effect. When the air is very humid (quite a common occurrence in Lochaber) much less evaporation occurs and the snow will stay soft.

Some wind also helps as this ensures a continual supply of dry air. This helps increase evaporation, just like how your washing dries quicker on a windy day when hung out, and more evaporation means more cooling of the snowpack.

Hopefully this helps explain the process and why there is more than just the temperature involved in changes to the snowpack.

Carn Mor Dearg with Ben Nevis behind.

Coire an Lochan, Aonach Mor looking South.

Glide cracks in Coire an Lochan. This can be a sign of instability but in this case I suspect that the snowpack will continue to slowly glide down the hill.

Looking down Easy Gully, Aonach Mor.

An icy crust on wet ground at 800 metres.

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