UPDATE: We've issued this video which shows the stats below, but quotes climatologists saying that it has been generally decreasing since 1900 worldwide:

http://c.brightcove.com/services/viewer/federated_f9?isVid=1

ORIGINAL BLOG: Every year we, the media, dust off the "NCDC Historical Chances of a White Christmas Map" (the AccuWeather version of which is shown below). We've been doing it so long now, I wondered how up-to-date the data was. It turns out, it was created by NCDC in 1995 based on the 1969-1981 climate normals, meaning some of the data was over 50 years old!

NCDC defines these 30-year periods as the "normal" or "average" temperature that you hear quoted by meteorologists. Every ten years, they recalculate them, so these stats were two versions behind. We had our climatologists recalculate them for major U.S. cities. Here's what we found for current probabilities of having 1 inch snow depth on Christmas morning (with the disclaimer that I have not personally vetted these numbers):

Of these twelve cities, half saw a reduction in their chance of a White Christmas, but 33% actually saw an increase in snowier times (check marks above). The cities losing more of a chance of a White Christmas were Philadelphia, Raleigh, Denver, and Chicago, all losing 3%, Richmond, VA at -4%, and the worst was Omaha, Nebraska, which now has 11% less of a chance of a White Christmas than it did in 1990.

Better news for the kids and the young at heart in Cincinnati, OH (+2%), New York City (+3%), Columbus at +4%, and finally Saint Louis, Missouri has seen its chance increased by 10%. Of course, people define White Christmases differently, and none of this "average" stuff means much when you already have a forecast for this year.

Why do we dream of a White Christmas anyway? Because all Christmas Lore was invented during the Little Ice Age.

2000_Year_Temperature_Comparison_xmasnewfinals

Given the hype about Global Climate Change killing White Christmases in 2008, I would have expected the numbers to be more negative, but of course in this study we're talking neither Global (only select U.S. cities) nor Climate (only 30 years of data for one day).

What's going on outside of the U.S.? Last year, I asked the opposite question (were they getting more frequent) and looked at the United Kingdom's statement. They say that, while White Christmases there are more rare than they were during the 1800s, they are not disappearing (in fact they had one almost every year between 1993 and 2001).

In Canada, they did a similar study with 18-year normals and more cities (I'm not sure which period the map above is for, and I can no longer locate it on Environment Canada's website). Their study reported that 71% of cities examined had less of a chance of a White Christmas in 2009 vs. 1982, while 20% stayed the same and 9% increased (something ignored by the media). It may be worth noting that all the increases were with cities near the coast (Victoria, Vancouver, and St. John's). That was not the case in the U.S. study.

*Washington, D.C. had to be excluded from my percentage calculations because it wasn't clear which climatological station there was used in the original NCDC survey.

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