Why does the heated needle probe method underestimate snow thermal conductivity?

Thanks to its ease of use, the heated needle probe method is broadly employed to measure snow thermal conductivity, both in the field and in laboratory. However, recent studies have highlighted that when compared to other measurement techniques, the needle probe shows a systematic underestimation bias. Here, we examine the theory at the base of the needle probe method and show that for a light and insulating material such as snow, the standard measurement protocol using heating times around 100 s leads to underestimations, as observed. Moreover, the damage done to the snow microstructure when manually inserting the probe leads to a further underestimation, that can exceed 50%. Nonetheless, needle probes remain the only easily deployed technique to measure snow thermal conductivity in remote areas. We thus propose a new measurement protocol to correct this underestimation and to obtain reasonably reliable values of snow thermal conductivity.