Are snow crystals randomly oriented or ordered?

Ice crystals are special: they can be much easier shifted along its hexagonal plane (perpendicular to c-axis) than perpendicular to it (see scheme on the right). Already in the past mid-century snow scientists at Weissfluhjoch assumed that the settlement of the snow cover is influenced by the orientation of the snow crystals. Due to the lack of a sufficiently precise instrument this context could not be proved up to now.

Fabienne Riche and Martin Schneebeli investigated together with Maurine Montagnat of the Laboratoire de Glaciologie et Géophysique de l’Environnement (LGGE) in Grenoble, the snow microstructure and the crystal orientation of natural snow during its metamorphism in the cold laboratory. In regular time intervals the snow texture was analysed by micro-computed tomography and by the Automatic Ice Texture Analyser (AITA). AITA is an instrument that was developed for a precise and straight forward measurement of the c-axis orientation in crystals. It could be shown that the snow crystals were re-oriented during metamorphism. Initially, c-axis were mainly vertically oriented and later they were horizontally oriented. Simultaneously to the new c-axis orientation the snow crystals also changed their form (Fig. 2).

These results show that the snow crystals are not randomly oriented in the snow cover as it was very often assumed. Up to now, this observed strong change in c-axis orientation has been predicted only theoretically. Further experiments will investigate the influence of crystal orientation to the snow settlement process [Riche et al., 2013].

The determination of c-axis orientation can also be carried out by another method. The computer-aided polarization microscopy (CIP) is a method based on standard optic and image analysis techniques. A slightly changed polarization microscope, a digital black-white camera and a computer is needed. This project of Sabine Leisinger will use CIP for the first time on snow [Heilbronner et al., 2014].