Of course, when associating "winter" and "Canada" we naturally assume things are bitterly cold. How cold is it? Let's look at this infrared satellite image from this morning:
|Fig 1 -- GOES-E infrared satellite image from 1745Z, Dec. 22, 2010. From the HOOT website.|
But note in the above image how most of Canada is also in the green and yellow color range. In looking at the loop of satellite images, we can see that these areas aren't really moving. Is all of Canada stuck under a very cold stratus deck? Not at all! That's actually the radiation coming from the surface. The surface is so cold that it's as cold as a lot of the clouds we're seeing in association with storms around the edges of the continent. Looking at the color bar again, we see that those colors should correspond to temperatures in the -20 to -30 degrees Celsius range at the surface. Is this accurate?
|Fig 2 -- "Current Conditions" around Canada at around 18Z, Dec. 22 2010. From the Environment Canada website.|
Interestingly enough, though, this cold air at the surface does not translate to "cold" air aloft. Here's the latest hemispheric analysis:
|Fig 3 -- Hemispheric analysis of MSLP (contoured) and 1000-500mb thickness (shaded) at 12Z, Dec. 22, 2010. From the HOOT website.|
But this isn't totally unexpected. We are under a very broad ridge in the central part of the country and you can see that translating to a very sprawling surface high pressure center across much of the plains on the image above. Ridging aloft with high pressure below indicates large-scale subsidence. However, initially to have a ridge there had to be higher heights which (roughly, as stated above) correspond to "warmer" air since warmer air tends to occupy a greater volume than colder air, vertically expanding the troposphere and lifting heights. Furthermore, the large scale subsidence inhibits cloud formation, and with snow-covered ground free to radiate to the open sky (particularly at night), the ground will cool off rapidly. This already stable situation then becomes even more stable as we see colder air near the surface and "warmer" air aloft--a stable temperature profile. All this makes for some pretty quiet conditions (and cold temperatures at the surface!).
There's a lot of hand waving in that argument above. The details regarding the warm temperatures aloft changing heights could be debated in terms of quasi-geostrophic theory, but I'm not going to go into that now. I just wanted to point out the cold temperatures visible in the IR image and how it translated to clear skies and cold temperatures for much of the middle of the continent.