3
$\begingroup$

I had a physics professor that said that it becomes colder after snow because heat from the air is absorbed by the snow as it melts.

This makes sense to me in that the snow should lower the temperature from what it would have been had the snow not been on the ground because the phase change of the snow should absorb ambient heat, but it does not make sense to me in that the ambient temperature should be lower than when it was snowing in an absolute sense, because the temperature would have to be > 0 C for the snow to melt and the snow would have to have been falling at a temperature < 0 C.

Does anyone know if temperature is lower following snow than it is during snow and if it is lower whether my professor got the mechanism wrong? Or, if there is something that I'm not accounting for?

Alternatively, does anyone know of good atmospheric data in the United States that I could use to confirm or disprove the correlation? (I would need temperature, precipitation, and whether it was snowing or not - more granular the better - I have strong python / data science skills and could do the confirmation myself).

$\endgroup$
4
  • $\begingroup$ I am afraid, the sentence of your prof, as you remember it, lacks concretization. - Are we talking about the daytime under sunlight? (what effect is more important? - radiation or reflection of the surface?) - Is the temperature above zero, close to it, or deep below? (Does the snow melt at all or melt in some moments?) - What is meant by "after snow" - after the snowfall stops, or after the snow cover melts? $\endgroup$
    – Gangnus
    Jan 18 at 16:53
  • $\begingroup$ - What is the wetness of the air? In Eastern Europe the weather with snowfall is warm and wet, the snow falls at temperatures of about +3- -5C. And the clouds fleeing means sunny and frosty high-pressure weather comes. On the contrary, in the dry areas, for example in inner Finland, the snow easily falls at -15C. And after the snowing clouds flow away, the sunshine comes and the temperature rises. $\endgroup$
    – Gangnus
    Jan 18 at 16:53
  • $\begingroup$ So, it is impossible to answer to such undefined question. $\endgroup$
    – Gangnus
    Jan 18 at 19:29
  • $\begingroup$ "It has a higher infrared (Droid was talking about it) emissivity than bare soil" YES. Please read Section 7 of the article that you can find at: doi: 10.1098/rsta.2018.0161 To demonstrate the contrary, please convince me that infrared emissivity of bare soil is above the one of snow, which is 0.98-0.99, and I would be very happy to change my idea. As Droid mentioned, color and emissivity in the IR are totally unrelated. $\endgroup$
    – Husky
    Jan 20 at 16:07

2 Answers 2

4
$\begingroup$

I think you're mixing two things together. There are generally 2 processes that correlate low temperatures during and after a snowfall, but they are separated. These are both based on physical laws but also on my personal experience :)

First of all, during a strong snowfall (the same usually does not happen for light snowfall) the snowflakes have to fall through different layers to finally arrive at the surface. If the temperature at the surface is not extremely low (and this is the case for most mid-latitudes snowfalls) most of the snowflakes will melt, at least partially, when falling. This melting process is what causes a sudden drop of temperature that can be observed during strong snowfall.

To observe this you need to get temperature data with high sampling (at least 5 minutes) because the process is really fast and hourly observations will not capture it. Plus, it is not always visible because it really depends on the temperature you have at the beginning of the snowfall. If you're starting from 1/2°C, you'd see a strong drop to values close to 0. If you're already starting from temperatures close to 0 or negative probably you won't see a change at all.

Atmospheric vertical profilers can also catpure snow melting and associated temperature drops, but this kind of data is obviously hard to find.

Second, the process of surface cooling after a snowfall is mostly related to radiative balance. In fact, after a strong snowfall usually the sky clears up (this is because again in mid-latitudes strong snowfall are associated with cold fronts moving in and bringing dry and cold air from the north). This enhances surface cooling because there are no clouds reflecting the infrared radiation irradiated towards the sky. Snow (especially when fresh) is really effective in radiate energy, way more than bare soil, and that's why the surface temperature decreases rapidly. Also, during the day, snow reflects a lot of incoming solar radiation, preventing the surface from heating up. If the snow persists for many days this process amplifies and can lead to surface thermal inversions. There are some other processes causing cooling at the surface, like snow sublimation and snow melting.

$\endgroup$
10
  • 1
    $\begingroup$ The white surface is NOT effective in radiating energy. The black is. You mixed radiating and reflecting capabilities. White reflects but does not radiate, and vice versa. So, the snow surface saves energy, in the ground and the snow itself. $\endgroup$
    – Gangnus
    Jan 18 at 16:40
  • $\begingroup$ The fact that the snow is "white" does NOT mean it cannot radiate: we're talking about thermal infrared spectrum so the snow radiates proportionally to its temperature and emissivity (Stefan-Boltzmann). As snow has a higher emissivity than bare soil it can emit more efficiently heat. You mention that snow is a good insulator, and this is another factor hindering surface warming because the heat coming from the soil does not reach the air but stays in the snowpack. There are other secondary effects like latent heat and a higher thermal mass that causes an additional cooling effect. $\endgroup$
    – Droid
    Jan 18 at 20:50
  • $\begingroup$ There are many papers describing these effects, you won't have a hard time finding sources otherwise I'd be happy to send it to you ;) $\endgroup$
    – Droid
    Jan 18 at 20:51
  • $\begingroup$ I have to say, though, the biggest impact on surface temperature may be just a side-effect of a snowfall, in the sense that, as I said before, such processes are associated with cold air advection which brings also really dry and clean air, a medium that is ideal to irradiate heat. So eventually the biggest factors enhancing surface cooling are the shortwave reflection during the day and the enhanced cooling due to drier air. All the rest is likely secondary $\endgroup$
    – Droid
    Jan 18 at 20:54
  • $\begingroup$ But at any rate, please, correct that: "Snow (especially when fresh) is really effective in radiate energy". It is absolute nonsense. It is effective in reflecting, not radiating $\endgroup$
    – Gangnus
    Jan 19 at 13:21
3
$\begingroup$

The aftermath of a snowfall can also cause a cooling near the surface, if an air mass moves in that's warm enough to melt the snow. The snow indeed starts to melt, thereby absorbing the latent heat required to do so and cooling the otherwise warmer air near the surface. If, as often happens during the winter, the warmer air also brings in a dew point above 0°C/32°F, then this near-surface cooling can lead to water vapor condensation and thus induce fog.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.