Snow Surface Temperatures
10th February 2025
A cold day with a strong East wind on Aonach Mor today. It was clear to the North-West but cloudy overhead. Little change in the snowpack, it continues to very slowly consolidate. Having said it did feel like it has now stabilised enough to be at the top end of the low hazard category rather than the bottom end of the moderate hazard category.
Looking down the Goose Gully. To the North and West there were plenty of clear blue skies.
The crags of Coire an Lochan. Conditions have not changed much in the past few days.
Looking over to the summit of Ben Nevis.
Looking to the South over the Mamores and Glen Coe hills in the background. It was a lot cloudier looking in this direction.
Approaching the locations of Saturday’s snow pit. Today’s snowpit location was adjacent to that used on Saturday. In that time, and despite the strong temperature gradient, there had been some consolidation.
Yesterday’s snowpit. Notice the strong temperature gradient and low snow surface temperature (-9.3C).
The Aonach Mor summit temperature over the past five days. Although it has been cold, the minimum temperature of the past five days has been about -7°C and that was this morning. Also note that the relative humidity has been 100% all day yesterday, which would imply it was cloudy throughout the day, which is not the case. In the winter the humidity as measured by the SAIS Aonach Mor Holfuy station reads a bit high, perhaps due to ice on/around the sensor.
Yesterday, the summit temperature was between about -4°C and -6°C and the snow surface temperature -9°C. Today the air temperature was slightly colder around -6°C, and the snow surface temperature also around -6°C. Why was the snow so much colder than the air yesterday, but they were the same today?
The reason is that there are several energy sources and sinks for the snowpack besides the direct exchange of heat with the atmosphere. Latent heat flux is produced by the exchange of moisture with the atmosphere. If the air is dry, and ice is sublimating (or water evaporating if the snow surface consists of both water and ice), this has a cooling effect on the snow surface. This is the process which means that in dry air the wet bulb temperature is lower than the actual air temperature. Alternatively, if the air is moist, it is possible for water vapor to condense on the snowpack and release heat, which has a warming effect on the snow surface.
But there’s more to the story than direct warming/cooler by the atmosphere and latent heat fluxes. This is radiative warming/cooling.
All normal matter emits electromagnetic radiation when it has a temperature above absolute zero. The radiation represents a conversion of a body’s internal energy into electromagnetic energy, and is therefore called thermal radiation. The snow surface constantly emits thermal radiation (heat) to the surrounding atmosphere. When the sky is clear, that heat escapes into space, allowing the snow to efficiently cool. If the sky is cloudy, there is significantly less radiative cooling. Cloud cover re-radiates heat back toward the snowpack.
Conversely, all normal matter absorbs electromagnetic radiation to some degree. In the visible spectrum, snow is good at scattering light. That is why it is white, and on a sunny day, with all that visible light from the sun is being scattered, why it is much brighter if there is snow cover than if there is not.
However, in contrast to radiation in the visible bands of light, long-wave radiation (which is invisible to our eyes, but can be picked up by the likes of an infra-red camera) is almost perfectly absorbed by the snow. An object that absorbs all radiation falling on it, at all wavelengths, is called a black body. Hence, snow is nearly a black body for long wave radiation. Although I don’t have the time (or knowledge) to go into the implications of this in detail, the fact that snow is close to being a black body for longwave radiation allows various predictions about the radiative energy balance of snow which are very important when trying to model climate in polar/mountainous regions.
A couple of nights ago it was clear and relatively calm. This allowed energy, in the form of blackbody radiation, being emitted from the surface of the snowpack to pour off into space. There were few clouds which would have absorbed and then re-emitted the radiation back to the snow surface where, being long wave radiation, it would have just been absorbed again. This, perhaps combined with a little bit of latent cooling due the relatively dry air, allowed the snow surface to cool to a lower temperature than the actual air. In fact the cold snow surface would then have started to cool the air close to the ground. A wind would have mixed the layers of air up, warming the snow surface up. However, the relatively calm conditions of a couple of nights ago would have allowed the snow surface and air close by to cool down.
Last night was much windier than the night before, there was also cloud cover last night. This would have prevented much radiative cooling, and even if this effect did occur would have been removed by the strong winds mixing the air.
As well as being an interesting effect in itself (as well as affecting little things like the earth’s energy budget), what is the effect of a low snow surface temperature? Well it does affect processes on and within the snowpack that can affect stability. However, discussion of these will have to wait to a future blog post.
The temperature (above) and wind(below) at the CIC hut a few nights ago. Notice that when wind dropped for 20 minutes around 3.30am, then so does the temperature. Was this due to cold ground, (being colder than the air due to radiative cooling) being able to cool the lower few metres of the atmosphere for the short period when there was not wind. Alternatively, was it that the temperature sensor itself was trying to cool down, and only when the wind died down did it stop being warmed by the wind? Either option is possible, but in both cases radiative cooling was the ultimate cause.
Comments on this post
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Simon
10th February 2025 5:39 pm
A very good explanation, thankyou very much
Ali Rose
10th February 2025 6:19 pm
Great blog post. really appreciatte these
Colin
10th February 2025 7:18 pm
Thanks for all the detail you put into your blogs. It makes for very interesting reading and added intrigue when out on the hills. Keep up the great work.
Ruth Love
10th February 2025 11:06 pm
Explains a lot. Thank you. I’m still confused about whether to try and go climbing Thursday/Friday. Seems like cloud and little base laid down, does not bode well. Maybe an easy solo to keep things ticking over.
lochaberadmin
11th February 2025 9:55 am
If we had had reasonable and ice snow cover before the thaw, then I suspect conditions could be very good at the moment. However, we didn’t, so despite a bit of snow things are very dry. The water ice routes will be building in the cold temperatures. Not great, but I would rather cold and dry than warm with rain!
Alistair Docherty
11th February 2025 10:24 am
Great post. This effect must make a huge difference to the global energy budget when added up over all of the snowy places in the world. Just another reason why we need to keep the snowy places snowy to prevent big global temperature swings. This stuff is useful to understand when hunting for nice spring snow to ski on vs neve. We need to rely on the sun to soften the snow on those dry clear days as the radiative cooling turns the snow back to neve in the shade / night. Interesting stuff.
Ruth Love
11th February 2025 2:02 pm
Many thanks again. Very useful knowledge imparting. Much appreciated.