December 27, 2013

Snow Makes It Cold

Snow is nature's refrigerator.  In fact, in places where snow can fall, record cold temperatures inevitably occur when there is snow cover.

You want to see snow's profound impact on surface air temperatures?

Here is an analysis of surface temperature at 6 AM PST today.
Cold temperatures in Canada and over the Rockies, but do you see the strange triangular tongue of cold temperatures extending from the Great Lakes towards Kansas?  What could that be?

Here is the answer!   The next image shows you the national snow depth analysis produced by NOAA. You see the triangular tongue of snow that is positioned over the cold area?

Let's take a look at a high-resolution NASA MODIS satellite imagery from yesterday. You can clearly see the bottom part of the white triangle is snow (can make out the terrain features).  The white stuff to the north are clearly clouds.

Going back to temperatures, here are the minimum temps last night over the region.  Teens in the snow triangle, but 20s surrounding it.

So why does snow produce cold surface air temperatures?  And remember surface air temperatures are measured at 2 meters above the ground level.

First, snow is highly reflective, allowing it to reflect the warming rays from the sun back to space.

Second, snow is a very good EMITTER of infrared radiation, much better than the atmosphere.  Such emission of infrared radiation to space produces substantial cooling...think of a refrigerator coil at the surface.

OK, so snow prevents the sun's rays from warming and snow cools by emitting infrared radiation.

But there is more.    Snow is a good insulator.  The ground and particularly the subsoils can be relatively warm.  So the snow prevents the conduction of heat from below.  That contributes to cooling.

And finally, if air temperatures get above freezing, melting snow stays at 32F until it gone.

There is a wonderful Northwest example provided by Lance in the comments below:  the very low temperatures experienced in Eugene, Oregon earlier this month.  Here are the minimum temperatures on the morning of Dec 8th.  Eugene is amazingly cold (-9F), while Portland was a balmy 12F).  What was the difference?  SNOW.

 Here is the MODIS satellite image from the day before.  Little or no snow around Portland, but you can see snow over Eugene quite clearly.    And the -27F observed in Redmond, Oregon that morning...over snow, of course.

So if you want cold temperatures you want snow on the ground.

I wish we had more here in the Northwest this year....

 P.S.  Thanks to Mark Albright for bringing up this topic...

Fracking and Ozone

UW Professor Becky Alexander has established a page on the Microoryza crowdfunding web site that outlines her project  to understand why natural gas fracking often leads to high ozone values over snow (go here to see it). If you want to learn more about this important project and how you can help it happen, check out the web site.


  1. Thanks for confirming what I was pretty sure was true. During the recent cold snap, the temp here dropped to 11 one evening... but I saw it as low as 6 back in 2008 when we had a couple feet of snow on the ground, and that, while cold, wasn't as cold as this cold snap as far as I could tell.

    And I too wish we had more snow!

  2. A perfect recent example of this in the NW would be on Dec 8th when the temp hit -10F in Eugene while Portland only dropped to 12F above that morning.

  3. On a completely unrelated note...

    Take a look at the UW timelapse video for Christmas Day -

    At the very end, just as it's getting dark, there is a wonderful train of Kelvin-Helmholtz waves in a patch of low clouds. A nice extra gift from Santa Clouds!

  4. Cliff, awhile back you posted about bad science and faulty assumptions. Can you tell me where that is or forward the post to me? Thank you.

    Donna Maher


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

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