Snow doesn't melt if the temperature is below freezing. However, there are several ways in which snow can disappear at temperatures below freezing. Parts of a snowbank which are above freezing will melt while the rest of the snow does not. A layer of dirt can absorb heat and melt the snow beneath it while the air temperature remains below freezing. Low humidity can cause snow to sublimate (pass directly from solid to vapor without melting into liquid) like dry ice at subfreezing temperatures.