Q: The other day we had a light covering of snow on the ground in the morning, but it seemed to melt away in the afternoon despite temperatures barely reaching into the 20s? How does this happen?
A: This is a fairly common question that is asked each winter. It makes sense that if the high of the day is below freezing, the snow should not go away. But does it? There are two things that happened to the snow during that cold day early this month. The first reason for the snow to disappear is the sun. The sun in the beginning of March is quite strong, about as strong as early October. And while you can really feel its warmth on your skin, the sun really doesn’t heat the air up directly. It does transmit a lot of energy through the air to the ground, which then warms the air through touch. So on the day in question, while the air did not warm up more than temperatures in the 20s, the sun did give the snow enough energy to help with some melt.
The second part of the answer would happen whether there is sun or not. If it were a puddle instead of snow, we would easily jump to the conclusion that the water would disappear through evaporation into water vapor. Ice and snow also changes to water vapor, a process which is called sublimation. Wind can speed up the process of evaporation and sublimation and that early March day was not only cold, but quite windy.
Q: Is the sun closer to Earth in the summer and farther away during winter? It was a topic of conversation today, and I’m pretty sure I learned it that way in high school but wasn’t sure.
Joe & Marie
A: Hopefully it is your memory that tricked you, and not that your high school teacher taught it that way. We’re actually 3 million miles closer to the sun in early January than we are in early July. While that number seems high, it represents only about 3 percent of the total distance from the sun, which is fairly negligible when it comes to our seasonal temperature change. What causes the change of seasons is the tilt of our Earth’s axis, which points the northern hemisphere more to the sun during the summer and away from the sun during the winter. This greatly changes the angle of the sun’s energy as it strikes the Earth, which changes the efficiency of the sun’s warming of the ground.
Now the interesting question that comes to mind is what would happen if the distance scheme was changed so that we were closer in July rather than January while the angle of the Earth’s tilt stayed the same. In that case, our seasons would be the same, but our summers would be slightly hotter and winters slightly colder. Interestingly, the shape of the Earth’s orbit and the tilt of the Earth’s axis do change, but over the span of tens of thousands of years, so you don’t have to worry too much.