Air Temperature and Radiation Depressions Associated with a Snow Cover

Abstract

An analysis of air temperature and radiation regimes an days with and without a snow cover at the St. Paul, Minnesota, climatological observatory was made based on a 16 December-15 March 23-yr temperature record and a solar and longwave radiation record for 11 of those 23 years. In addition, an overlapping 41-yr temperature record of the Minneapolis-St. Paul National Weather Service Station (MSP) was analyzed for corroboration of the St. Paul temperature results. It was found that both the average maximum and average minimum air temperatures for winter days with a 10-cm or greater snow cover were 8.4°C lower than on the snow-free days. For days with intermediate-depth snow (>0 and <10 cm deep) the depressions of the maximum and minimum temperatures averaged about 2°C less. The temperature depressions at MSP were about 2°C less than at St. Paul for both snow-cover depths, a difference believed to be due to the more urban surroundings at MSP. A difference in the depression of the winter month temperatures was observed at MSP but not at the St. Paul observatory. The St. Paul results were unexpected, since it has been suggested that a greater maximum temperature depression, due to a higher sun, would occur in March than in December. The air temperature depressions compare favorably with the mean 16 December- 15 March radiometrically determined surface temperatures, which indicated that the intermediate snow depth and the ?10-cm snow depth were 1O° and 15°C, respectively, colder than the surface free of snow. The mean longwave radiation loss was 3.94 MJ m?2 day?1 greater from the snow-free surface than from a 10-cm or greater snow cover.