– On a sunny day, why does snow melt on the sidewalk even when the temperature is well below freezing?
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Temperature is measuring air temperature, not the surface temperature of things. So one reason is that the sun can heat things up above freezing and cause melting.
Snow, on top of other snow, is able to stay cooler because as the sun hits the top layer a lot of it is reflected back (snow is white and shiney). The snow that is heated by the sun above freezing can melt and then refreeze when it hits the snow underneath it. That’s why snow often develops an icy crust on top – the melt/refreeze cycle.
When the sun hits a sidewalk on a freezing day it can be heated up without melting (unlike snow) so that top layer gets warmer. Concrete absorbs heat pretty well so it can get above freezing and hold that thermal energy. It doesn’t have to get that warm either. Sidewalks are generally cleaned off after snowstorms so, when snow gets knocked off the edges (or a bit is left on the edge) the sidewalk has enough energy stored up to melt the snow.
You will notice this only happens on days that are just below freezing or just really sunny days where the sidewalk is rather exposed.
Salt is another possible reason – since salt lowers the freezing temperature of water, and people often sprinkle salt on sidewalks in suburbs or cities.
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