The study includes physical properties of the material as it changes, bulk properties of in-place snow packs, and the aggregate properties of regions with snow cover. In doing so, they employ on-the-ground physical measurement techniques to establish ground truth and remote sensing techniques to develop understanding of snow-related processes over large areas. Measurement and classification In the field snow scientists often excavate a snow pit within which to make basic measurements and observations. Observations can describe features caused by wind, water percolation, or snow unloading from trees. Water percolation into a snowpack can create flow fingers and ponding or flow along capillary barriers, which can refreeze into horizontal and vertical solid ice formations within the snowpack. Among the measurements of the properties of snowpacks that the International Classification for Seasonal Snow on the Ground includes are: snow height, snow water equivalent, snow strength, and extent of snow cover. Each has a designation with code and detailed description. The classification extends the prior classifications of Nakaya and his successors to related types of precipitation and are quoted in the following table: All are formed in cloud, except for rime, which forms on objects exposed to supercooled moisture. It also has a more extensive classification of deposited snow than those that pertain to airborne snow. The categories include both natural and man-made snow types, descriptions of snow crystals as they metamorphose and melt, the development of hoar frost in the snow pack and the formation of ice therein. Each such layer of a snowpack differs from the adjacent layers by one or more characteristics that describe its microstructure or density, which together define the snow type, and other physical properties. Thus, at any one time, the type and state of the snow forming a layer have to be defined because its physical and mechanical properties depend on them. Physical properties include microstructure, grain size and shape, snow density, liquid water content, and temperature. When it comes to measuring snow cover on the ground, typically three variables are measured: the snow cover extent (SCE) — the land area covered by snow, snow cover duration (SD) — how long a particular area is covered by snow, and the snow accumulation, often expressed as snow water equivalent (SWE), which expresses how much water the snow would be if it were all melted: this last one is a measurement of the volume of the snowpack. To