The Impact of Snow Cover on Surface Temperatures
A narrow, but intense snow band formed over an area stretching from central Iowa to southwestern Wisconsin over the the night of 19 February into the morning of 20 February. Localized accumulations reached a foot in parts of northeast Iowa. This map from the La Crosse, WI, National Weather Service office shows the extent of this snowfall. (A lengthier discussion of this event from NWS La Crosse is available here).
A week later, this narrow corridor of snow is still around and still exerting influence on local weather conditions. Take a look at the following plot of surface temperatures from AWIPS in the early afternoon of 27 February 2026. North-central Iowa is experiencing temperatures in the lower 60s while parts of eastern Iowa are even reaching 70. However, there’s a cooler alley in northeastern iowa where temperatures are in the low-to-mid 40s.
Without any context, an analyst might opt to put some sort of synoptic-scale feature in northeast Iowa and assume that larger-scale flow is causing this pool of cooler temperatures. However, this being the CIMSS Satellite Blog, we’re going to recommend that you check out contemporaneous satellite imagery to verify what might be happening. Here’s the same figure, this time with the 0.64 micron highest resolution satellite imagery included as the base layer.
It’s clear that the surface snow cover is exerting a downward influence on temperatures. But why? There are two main reasons. The first is that snow has a high shortwave albedo: it reflects a substantial fraction of the light that shines on it. The albedo of new fallen snow can be as high as 90%, thoough for older snow such as this it’s much closer to 40 or 50%. Still, that is a substantial fraction of the incoming solar heating that is redirected away from the surface and cannot help to change the temperature. The other impact of snow is through phase changes. The solar energy that isn’t reflected from by the snow goes into changing its phase through melting or sublimation.
Here’s a pair of animations captured from AWIPS (thus the color dithering is not ideal). The first shows the true color RGB with the surface temperatures, while the second depicts the Day Cloud Phase Distinction RGB. Both are useful for identifying the location of the snow band surrounded by bare ground, while the latter animation helps discriminate between the snow (green) and ice clouds aloft (pink).
Finally, we should also take a look at the impact of the snow band on nighttime temperatures. While the earth’s surface generally has very high infrared emissivities, surface snow has a near perfect emissivity up to 0.99. This means that the snow is a very effective emitter of infrared radiation, more so than other surface types. Thus, we expect snow-covered ground to be colder at night than uncovered ground. This, of course, is on top of the fact that the snow-covered ground didn’t get as warm during the day as the other locations did, so it’s already starting from a lower temperature. This image shows the Band 13 (10.35 micron) imagery for 12:40 AM on the 27th. The color table has been adjusted to enhance contrast. Recall that for this window channel, in the absence of clouds we’re reading the brightness temperature of the surface. That surface is colder where the snow is, and the surface temperature observations back that up.
