Compare the volume of snow with the volume of water that forms after it melts.

The volume of melted water will always be less than the volume of snow that has been melted. During the formation of a snow mass, significant voids are formed, filled with air, they occupy a significant part of the volume of snow. Ice is formed similarly, it has a lower density than water, does not sink, which is of great biological importance – ice-covered reservoirs do not freeze to the bottom (air is a poor conductor of heat), in the lower layers of water the optimum temperature is maintained for the life of its inhabitants.