Measurement of growth and density of dendrite crystals

The great variety of structure of individual dendrite snow crystals having six-fold symmetry has been realized for many years, clearly demonstrated by Bentley and Humphreys long ago. Laboratory studies have shown that detailed structure of such dendrites is related to ambient conditions through temperature, supersaturation and fall velocity. The density of these particles, required for estimating radiation scattering, terminal velocity, mass, and hence precipitation rate has not been well defined. Detailed measurements of dendrite shape from images, combined with simplified assumptions that dendrites are thin (10 – 50µm) compared with size (50 -5000µm) provides a measure of the local density. This may vary from a central density of 0.92 (compared with water) falling to zero at the periphery. Such measurements of the changing local density quantify the time history of the dendrite growth regime. A convenient simplification is a six-fold symmetry; three fold symmetry, sometimes as triangular crystals, does occur with a frequency of about 10%, suggesting the presence of stacking faults. Detailed analysis of the periphery - center density gradient characterizes both the dendrite growth process and ultimately, through integration, leads to particle mass.