I have made this observation on days in the winter in which there was snow cover: The air temperature was below freezing yet ice and snow was melting off house roofs, cars and other objects. Why would this occur? 1. Air is a very poor absorber of solar radiation while objects on the earth's surface are much better at absorbing. Even snow with its very high albedo and reflective ability is more absorbing of solar radiation than the air. While a temperature sensor exposed to the air may detect temperatures below freezing, the sun's radiation can warm individual objects above freezing (especially objects with a low albedo). 2. The temperature of the earth's ground surface and/or objects on the surface may be above freezing. The air temperature at the observing level may be below freezing while the temperature of air immediately surrounding certain objects may be above freezing. This can occur when the soil temperature has not yet adjusted to or modified the air temperature at the observing level. |