Spectral measurements of surface hoar crystals

Surface hoar crystals are commonon the surface ofmountain snowcovers.Onceburied, layers of large plate-shaped surface hoar crystals are prone to releasing dangerous snow-slab avalanches. Since snow microstructure influences the optical properties of snow, remote sensors could potentially detect the formation of surface hoar and other snow types associated with avalanche release. The spectral reflectanceof377 snowsampleswasmeasuredwith a field spectrometer between750and2500 nm, including161 samples of surface hoar.Morphological snow shapes associatedwith critical avalanche layers (surface hoar, near-surface faceted particles and depth hoar) had lower average reflectance factors than non-critical snow shapes at infrared wavelengths. Needle-shaped surface hoar was more reflective than plate-shaped surface hoar,but therewerenosignificantdifferencesbetweendifferent sizesof surfacehoar.Normalizeddifference indices calculatedwith reflectance from twowavelength bands is presented as a potentialmethod to classify critical snow surfaces remotely, althoughmelt-freeze crusts near the surface complicated the classification. Accordingly, further studying on the effect of melt-freeze crusts and quantification of the bidirectional reflective properties of critical snow types is needed.