Use of a thermal imager for snow pit temperatures

Abstract

Abstract. Weak snow of interest to avalanche forecasting often forms and changes as thin layers. Thermometers, the current field technology for measuring the temperature gradients across such layers -and for thus estimating the expected vapour flux and future type of crystal metamorphism -are difficult to use at distances shorter than 1 cm. In contrast, a thermal imager can provide thousands of simultaneous temperature measurements across small distances with better accuracy. However, a thermal imager only senses the exposed surface, complicating its methods for access and accuracy of buried temperatures. This paper presents methods for exposing buried layers on pit walls and using a thermal imager to measure temperatures on these walls, correct for lens effects with snow, adjust temperature gradients, adjust time exposed, and calculate temperature gradients over millimetre distances. We find lens error on temperature gradients to be on the order of 0.03 • C between image centre and corners. We find temperature gradient change over time to usually decrease -as expected with atmospheric equalization as a strong effect. Case studies including thermal images and visual macro photographs of crystals, collected during the 2010-2011 winter, demonstrate large temperature differences over millimetre-scale distances that are consistent with observed kinetic metamorphism. Further study is needed to use absolute temperatures independently of supporting gradient data.

Limitations of using a thermal imager for snow pit temperatures

“…As well, if thermal conductivity properties play a role, this will add more uncertainty in the interpretation of the thermal signal. Shea et al (2012c) found no relevant sharpening of gradients with exposure time. This finding is opposite to what is presented here.…”

“…Crusts showed the same surface energy balance process as artificial concavities. However, this does not prove that internal processes in an undisturbed snowpack are not existent to explain a warm crust, as done by Shea et al (2012c). The initial warm crust shown in Figs.…”

“…These cameras are identical to those used in Shea et al (2012c) and Shea et al (2012b), respectively. These cameras measure in a spectral range of 7.5 µm to 13 µm.…”

“…Thus, recent studies were promising (Shea and Jamieson, 2011;Shea et al, 2012b, c) where a thermal camera was used to image the wall in snow pits, which delivers a resolution of less than 2 mm. Shea et al (2012c) found melt-freeze crusts to be warmer than the surrounding snow. In an hourly measurement setup they presented a warm crust during cooling of the atmosphere.…”

“…Besides emissivity dependencies Shea et al (2012c) discussed an additional error source. During the assimilation of the exposed snow pit to air temperature, heat may be conducted unevenly from behind, depending on different thermal conductivity properties in certain layers, but they did not find a relevant sharpening of temperature differences (gradients) between pixels over exposure time, which showed them that conductivity differences did not play a major role in interpreting the thermal image.…”