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the formation of solid ice on the surface of a body or stream of water; the period during which there is a solid layer of ice on a river or body of water. Areas of rapid current or inflow of warm waters—for example, effluent or groundwater—may remain free of ice and are called lanes or polynyas.
The duration of a freeze-up and the thickness of the ice depend on the duration and temperature conditions of the winter, the characteristics of the body of water, the depth of snow cover, and the wind conditions (for example, on bodies of water of the Eastern European Plain, the ice freezing period lasts several days in the southern parts and up to six or seven months in the north; the thickness of the ice varies from 20–30 cm to 1 m or more, respectively). Small rivers and quiet pools freeze more rapidly than large rivers. Mountain streams usually do not freeze over because of the fast current.