The demarcation between fast ice and drift ice or between different zones of drift ice. When used as a climatologic term, for describing the position of the boundary of spreading of ice of any concent
ration or age in any given month or a period based on the observation data for a number of years, the tem should be preceded by a word mean, median, minimum or maximum with indication of the ice cover characteristic after it. For example: Median boundary of multiyear close ice.
Same as the wet-bulb temperature when the latter is below 0C and the water on the thermometer is frozen. Note that in dry conditions the wet bulb of a psychrometer may freeze when the air temperature
is appreciably above the freezing point.