Conditions external to a snow pack can influence its physical characteristics and transformation over time. Wind speed near the surface, for example, influences how crystals at the surface are packed. At high speeds, crystals can be dragged over the surface, reducing their size and making a more dense pack. Other conditions, such as rain on snow, temperature of the ground under the pack, and varying temperatures and humidity over time can also influence snowpack structure. A given snowpack often has layers that vary in character. Duration and rate or intensity of melting also influence a snowpack's characteristics. Elements influenced by these factors include density, specific gravity, water content, impurities, grain shape and size, pack temperature, and layer structure and composition. Based on the effects of these influences and variables, this article defines four classes of snow packs. Other sections of the article discuss the bulk and thermal properties of snowpacks, their mechanical and frictional properties, and their chemical properties. The discussion of the properties of snowpacks is followed by a detailed discussion of the selection of casting methods for evidence impressions in snow based on snowpack characteristics. The authors note that the properties of the snowpack can influence the effectiveness of various casting methods. They caution that many investigators have become too dependent on the exclusive use of aerosol wax products, followed by dental stone casting for recording impressions in snow. Although this method may produce excellent results under some conditions, other methods may be better suited for a particular snowpack. Understanding how snow conditions may influence the interaction with various casting materials will assist the investigator in choosing the best casting method. 17 references