Wind speed is another important variable. It is technically incorrect to say that the blowing air carries the cold away (just as centrifugal is technically incorrect) but has great explanatory force. Also the color matters. In early spring when a lot of the snow has melted, all the accumulated dirt in that snow is now on top and the black snow melts much faster. It was worse before stoop and scoop laws because you also had several months dog shit in the snow.