The sun's radiation is a powerful sublimation agent. You may have noticed that on a sunny day after a snowfall, snow remains in the shady places but quickly melts or sublimates in the regions exposed to direct sunlight. Overcast conditions do NOT prevent all the sun's energy from reaching the surface. If none of the radiation reached the surface it would look like night outside. Even in overcast conditions, the shady areas will have an advantage of retaining snow accumulation longer. Sublimation occurs when ice is converted directly to vapor without going through the liquid stage. Shortwave radiation from the sun is effective in forcing this sublimation process. This sublimation process is more powerful in the lower latitudes since the sun angle is higher. When temperatures climb above freezing, melting and sublimation act together to remove the snow / ice accumulation. Even when the air temperature is below freezing, shortwave energy can warm objects adjacent to the snow above freezing and this will melt snow / ice. Objects include the metal of cars and dark roofs. After the ice melts, the water will freeze again when it moves to a shady region (forms icicles) when the ambient air temperature is below freezing. A night snowfall has the potential of producing much more accumulation than a day snowfall, especially in lower latitudes. The advantages of a night snow include: (1) more likely temperatures will be cooler and stay cooler without being increased gradually by daytime heating (2) no shortwave energy to sublimate the snow or warm ambient air temperatures and surface objects (3) given the same temperature profile of the troposphere, a night snow will have a lower liquid content since no shortwave radiation is present (4) less traffic on the roads. These four advantages apply most significantly to a snowfall that occurs when temperatures are near freezing (i.e. a 32 F snowfall at 2 am vs. a 32 F snowfall at 2 pm). |