– On a sunny day, why does snow melt on the sidewalk even when the temperature is well below freezing?


– On a sunny day, why does snow melt on the sidewalk even when the temperature is well below freezing?

In: 1

Temperature is measured in the shade. But in the sun, sunlight heats surfaces to a higher temperature than the ambient air temperature.

What determines the air temperature is complex but we can mostly call it “the weather”.

The temperature of surfaces though adds in two more factors – light and mass.

Light is important because air is mostly see-through, meaning it doesn’t really absorb the light’s heat energy very well. But surfaces, especially dark surfaces like asphalt, absorb the light energy and get warmed up by it. So even it’s 20F outside, on a bright sunny day the roadway might be 35F from the light energy. That’s enough to melt the snow.

Mass is important too. Air doesn’t have much “stuff” to it, meaning it’s easy to change the air temperature. But the Earth has tons of “stuff” so it changes it’s temperature very slowly. So even on a 20F day by air, the ground might be 35F simple because it takes a long time to change the ground temperature, the sidewalk is essentially “still warm” from yesterdays’ 40F day even though the air has changed to be much colder today.

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.

The weather reports the ambient temperature, which is the average temperature of the environment or the air temperature.

Different objects can be warmer or colder than the ambient temperature.

The sun heats the ground during the day and it gets warm enough to melt the snow on it. Concrete is very good at absorbing heat which is why snow tends to melt on it before the ground.