One of the observations that I report every morning to the National Weather Service is whether or not there's any snow on the ground at The Franklin Institute.

Where there are no observers to report snow cover, meteorologists resort to satellite data. Here's a look at North American snowcover as it stood about three weeks ago. At that time, the snowline was still well north in Canada. By the first of December, cold air and several storms had pushed into the West and the snowcover expanded greatly southward. There was even a stripe of snow on the ground in Texas and Oklahoma. Now here's the most recent image, which now shows some snowcover in the Northeast from last weekend's storm.

The extent of a snowcover is more than just some interesting weather sidenote - it's also a forecasting tool. Cold air masses simply have a better chance of developing and maintaining themselves over a colder snowpack than over warmer bare ground.