WET BULB FREEZING LEVEL

Because evaporative cooling is so important in rain versus snow forecasting, the wet-bulb 0degC level is a better guide to snow-risk level than the actual ZDL. The wet-bulb is the lowest temperature to which air can be cooled by evaporating water into a sample, which is a mechanism often important in determining whether it will rain or snow at or near the surface. However, note carefully that advection, both cold and warm must be taken into account - the former is particularly important in marginal situations in the vicinity of a well-marked cold front. (see also "Downward penetration of snow" in this Glossary).