You might think this is a silly explanation, but I get this question a lot.
The answer is a little more complicated (but also easy to explain) than it sounds. The simple answer is: The snow keeps it cool, like putting ice in a cooler.
The real answer is the snow keep us cool through evaporational cooling.
Evaporational Cooling
In physics, things (in this case, air) that are warmer will always do work (heat up) on things (in this case, the snow) that are colder because of the second law of thermodynamics.
Imagine a world where cold things did work on warm things. So, if you put ice in a glass of lemonade in the summer, you had to drink it super faster… before it froze. And then the ice froze the glass to the table. And the table to the ground.
Our world would be a very different place.
Or as German scientist Rudolf Clausius said in 1850 when the Law was, in a sense, still a Bill:
“Heat can never pass from a colder to a warmer body without some other change, connected therewith, occurring at the same time.”
So when snow is on the ground, the air will always try to heat up the snow. But that takes energy. Because with physics it takes energy to do any work. This time, the “work” is changing water between it’s various forms (ice to liquid to gas).
And in this case, the energy is heat.
And so long as the air is warmer than the snow, it will continue to try to heat it up.
How Cold is the Snow?
Snowflakes start out in the Dendritic Growth Zone up in the clouds at a temperature of about -3F to 5F. Very cold. And in fact, that is a common misconception. Snow isn’t “made” when the temperature ont eh ground is near freezing. But that is for another post!
So! As long as the snowflakes are cooler than the air temperature, the air will do work on them to try and melt the flakes. And the warmer air must use its heat to do so. And in turn, it “gives” some of the its own heat to the snow.
And once it “gives” up heat, the air is cooled down.
But that’s only the half of it! In fact, it takes 7.5 times more energy to turn that now melted snow into water vapor.
The warmer air will then work to evaporate the snow, which is now water, “giving” even more heat to the water to do that! After it’s all said and done, the air temperature, because it has done all of this work, is now even cooler.
So, what?
This is the coolest (pun intended) part! For example, Let’s say you are watching a local TV weather forecast at 10pm, and it is snowing outside at the time and 25 degrees. Let’s also say the forecast low is for 20 degrees in the morning with a high of 40 the next afternoon..
You can assume, that so long as the snow was made in the DGZ (where it is very cold) and there isn’t some kind of crazy inversion (a warm layer in the atmosphere between the DGZ and the ground), that the forecast temperatures for the following day are MORE LIKELY to be colder than the forecast than it is to be warmer.
So as you make plans to wake up and get moving in the morning, you may want to prepare an extra layer of clothing.
That said, hopefully the forecaster on TV understands this concept, and has lowered the forecast temperatures for the following day to account for the additional cooling.
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