In terms of snow cover in North America, how does the first few years of the 2000s compare with the decade of the 1990s? Also, does Eurasian snow cover fluctuate in synchrony with North American snow cover?

Much of the U.S. has been blasted this week with wintry weather, cloaking a large portion of the nation from the Carolinas to the southern Plains in a mantle of white (see http://lwf.ncdc.noaa.gov/oa/climate/research/snow/recent.html). Here in the east, we've had to deal with a full week of temperatures continuously below freezing as well as snow, sleet, and freezing rain, which have conspired to close schools and businesses.

As of this past Tuesday (January 27), the continental snow cover area is slightly greater than the long term average for this time of year. Late January and early February is typically the period when the snow cover extent in the Northern Hemisphere protrudes furthest south. However, the snow mass or amount of water stored in snowpacks, usually doesn't peak until late winter (mid February through early March).

For the month of January, the average or mean snow cover area for North America is 17.0 million square km, but since 2000 (not including 2004), the January snow cover area has averaged just 16.5 million square km (from here on, M km2 will be used to denote "million square km"). For the decade of the 90s (1990-1999), January's snow cover area averaged 16.9 M km2 -- just about normal. During the 1970s, the average snow cover for January was 17.3 M km2, and for the 1980s, the mean was 17.1 M km2. By the way, decades are usually demarcated by the years starting with "1" and ending with "0." However, in order to squeeze out one more year for the 2000s, I altered the conventional decadal period by starting the decade with "0" and ending it with "9." Also, it wasn't until the early 1970s that snow cover was first measured reliably measured with satellites. The data used here is from 1973 onward.

Looking at the average annual snow cover (including all 12 months), the same sort of trend appears -- slightly less snow cover in each succeeding decade. Thus far this decade, the 12-month mean snow cover in North America is 10.1 M km2, compared to the long term average of 10.4 5 M km2. For the 70s, 80s and 90s, the mean annual North American snow cover was 10.8, 10.4 and 10.3 M km2, respectively.

Part of the blame for the milder winters and rather paltry snow amounts across much of the U.S. and North American since about 1990 has been attributed to global warming. It seems reasonable that if the global thermostat is turned up, then there's apt to be more rain and less snow. Of course, things are a bit more complicated than this as we shall see below.

In Eurasia, the long term average snow cover for January is 28.5 M km2, but for the last 4 years, the average has been a robust 29.3 M km2. The average snow cover extent during the 70s, 80s and 90s was 28.9, 28.4 and 28.0 M km2, respectively. In addition, the long term 12-month average annual snow cover for Eurasia is 14.2 M km2, which is close to the average during the past 4 years (14.1 M km2).

Because Eurasia is so much bigger than North America, and since considerably more of Eurasia resides in Arctic latitudes compared to North America, thus far in the 2000s, the average snow extent for the entire Northern Hemisphere during the month of January (45.7 M km2) is actually greater than in the previous two decades. However, the 12-month average snow cover for the Northern Hemisphere during the 2000s (24.2 M km2) is slightly less than the long term record (24.6 M km2). So since the 1970s, in regards to the average annual snow cover in the Northern Hemisphere, there has been a small decrease in snow coverage. Therefore, while it's evident that snow is reaching as far equatorward in recent Januarys' as in prior decades, it's not staying around as long. Increasing global temperatures haven't prevented snow from occurring, rather it seems that warmer temperatures in spring and summer have expedited the annual northward retreat of the snowline.

It's true that if we torture these numbers long enough, we can get them to say just about anything. Nonetheless, we're neither heading for another ice age, nor are we about to lose all of our snow in the immediate future. What's interesting, though, is that North America and Eurasia don't appear to be in sync in terms of snow cover extent in recent years. While in many years snow cover extent in North America and Eurasia correspond quite closely, in other years the extent of the snow cover between the two continents is completely out of phase. The reason for this largely has to do with upper atmospheric circulation patterns.

If you could look down at the North Pole from above, you might be able to notice that the polar jet moves in a serpentine fashion about the pole -- counterclockwise direction. Typically, four or five pronounced waves or dips in the jet stream can be observed at any given time. Where the waves dip into the mid latitudes, troughs of cold air push south, and where the jet arches toward higher latitudes, ridges of relatively warm air move north. Troughs and ridges are temporary features, but from one season to another they tend to have preferred locations. For instance, during a good portion of the past 15 months, the polar jet has been positioned further south over eastern North America than is normally the case.

It's not at all unusual for a trough to exist over eastern North America during the winter when a ridge is in place over western Europe while another ridge rests above western Northern America. When this happens, snow is likely to cover the ground in New England and the Middle Atlantic states, but countries such as France and Germany will be snow free.

So what does the rest of the winter portend? We'll have to wait until Monday to find out, but a lot of us are rooting for Punxatawney Phil to not see his shadow (http://www.groundhog.org/).

To view snow maps and charts visit the following web site: Northern Hemisphere Snow Cover http://www.cpc.noaa.gov/data/snow/