The global snow and ice cover or the solid phase of water forms the Earth’s cryosphere. Snow is a porous, soft form of ice appearing white in natural light. Snow and ice are dominant factors in the nature in cold regions, found in the atmosphere, on Earth’s land and water surfaces, and in the ground (Table 1.1). The polar ice sheets in Antarctica and Greenland contain by far most of the ice volume, while seasonal snow and sea ice form the largest visible area of the cryosphere on the Earth’s surface. The thickness and renewal time of the ice sheets are large, while seasonal snow cover and sea ice are thin with short renewal times. The frozen ground can be thin and seasonal or thick and perennial, and their area accounts for 35% of the Earth’s land surface. In the atmosphere, the lifetime of ice crystals is of the order of days.