Thermal Photography: Some Photos
This avalanche, size ~1.5, is about 5 minutes old. The bed surface is cooler than the surface because that is the temperature that the surface hoar -- the failure plane -- was when the avalanche slid. This implies that the snowpack is rather good at insulating its weak layers from temperature change.
These bushes, although cold when exposed above the snow surface, can conduct heat from the ground below and warm the snow from underneath. This heat conduction to very near the surface can form facets due to a steep temperature gradient between the bush near the surface and the cold night sky at the surface.
Although photos like these raise interest in the use of thermal photography for avalanche rescue -- i.e. being able to see a buried person -- remember that the bush has had all season to let its heat percolate up through the snow, a timeframe that is not very useful for avalanche rescue.
These facets form isothermal 'chains', or fingers of the same temperature reaching up or down depending on your perspective. These constructs are energetically favourable because they decrease the steepness of the temperature gradient from ground to sky. These facet chains can be found thermally even before striations are present.
Trees affect the snowpack around them in various ways. One of the ways is that trees often (but not always) prevent the growth of surface hoar around them within about 4 m due to a radius of heat that they create on the snow surface. Surface hoar needs a cold snow surface to grow, and so this heat radius can prevent growth. These photos were taken on the listed dates and times in Kananaskis on a cold night, about -25 C air temperature.
On the same night as the photo taken above, this photo was taken to demonstrate the ability of the snow surface to cool itself. First, we deduced that this heat was not due to friction by carefully pressing skis (boots, gloves, ski poles) into the snow rather than sliding along the snow. We obtained the same result.
At night, with no clouds, the snow tries to thermally balance itself with outer space as air is nearly transparent to escaping heat (as anyone who has gone outside without clothes in winter can attest). This type of longwave radiation escape is what cools the snow surface and enables the growth of surface hoar and surface facets. Just below the very surface, the snow retains the heat from the day because the surface snow, though cold, is a good insulator.
| Attachment | Size |
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| 5min_old_avalanche.png | 276.47 KB |
| bush_heating.png | 496.95 KB |
| facets.png | 337.36 KB |
| tree_heat_radius.png | 230.6 KB |
| warm_ski_tracks.png | 109.87 KB |
| march3time.gif | 30.64 KB |
| march3time_small.gif | 199.05 KB |
