C-axis orientation of snow crystals in snowpack

The aim of the project is to find out how crystals in snow are oriented.

Introduction

The physical properties of snow are tied to its microstructure. Especially for the slow, plastic deformation of snow, the crystal orientation could be an important factor in addition to the geometry of the ice matrix. While micro-computed tomography measures precisely the snow microstructure, it gives no information about the orientation of the ice crystals.

Theory

The Birefringence of ice permits to measure the orientation of the ice crystals c-axis. The c-axis is the crystallographic axis of the ice crystal perpendicular to the hexagonal structure of the ice. This hexagonal structure of the ice crystals are called basal plane. Traditionally, the c-axis of these crystals can be measured using a Rigsby stage, which is difficult for the small snow crystals and time consuming.

Snow is a porous material composed of ice and air. The orientation of the snow crystals can therefore also be determined. Automatic ice- fabric analyzers, such as the Fabric analyzer G50, allow to determine automatically the direction of the c axes of ice crystals.

Methods and Results

Production of thin section used to measure the c axis orientation. The thin sections are produced with a thickness of 120 µm to 150 µm.

A few steps are shown in figure 1: [left] microtome used to cut the snow sample, [center] melting of di-ethyl phtalate on a heat plate with a controlled temperature (-2°C), [right] the thin section is covered with a small glass for the analyzes.The Fabric analyzer G50 can measure the orientation of the snow crystal automatically.

figure 1: The preparation of thin sections is a several steps procedure, where every step has to be realized carefully in order to no destroy the snow sample.

After the preparation of the thin sections, the snow samples are immediately analyzed with the fabric analyzer G50, mounted with the “High resolution” camera that allows a resolution of 6.5 µm per pixel and an angular resolution of the c-axis orientation of 1 degree. (the U-stage has a resolution of about 5-10 degrees).

figure 2: The analyzer gives colored images of the snow crystals, every color correspond to a particular orientation. The relation of the orientation and the color is defined by a color wheel. The snow image shown is a thin section of depth hoar.