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