People who are still digging out from blizzard-like conditions earlier this month may find this hard to believe, but their chances of seeing a white Christmas in the years ahead are disappearing about as fast as hail stones hitting a hot griddle.
There has been a dramatic decline of the number of days of snowfall during the Christmas season over the past few decades, especially in the northeastern part of the United States. Warmer temperatures over that region have reduced the number of snow days by as much as 26 percent during the period of Nov. 25 to Dec. 24, while lots of folks are out hitting bricks trying to find that perfectly useless gift for grandpa.
The trend toward fewer days of snowfall has been documented by Dale Kaiser, a meteorologist in the Carbon Dioxide Information Analysis Center at Oak Ridge National Laboratory.
Kaiser studied the period from 1948, when the U.S. Historical Climate Network started collecting far more data on climate across the country, through 2001.
"We found significant decreasing trends in the number of snow days," Kaiser says. "Sometimes, it's like they turned off the spigot."
That's especially true for areas east of the Mississippi River, where 117 of 125 stations reported an average of five fewer days with snowfall. That may not sound like a lot, but some of those areas normally have few snowy days anyway, so five is a significant number, and the period covered by the study is only 30 days.
Conversely, an area encompassing the central Rocky Mountain states of Utah, Colorado and Wyoming, and extending eastward into Nebraska, experienced more days of snow during the same period. The popular ski resort areas around Provo, Utah, led the list with an average of 9.8 more snowy days, as if they needed it.
So, what's going on here? The results are consistent with global warming, but fewer snow days does not necessarily translate into less snow. A whole bunch of snow can fall in a single day, and that counts just as much as a thin dusting of flaky white stuff.
"One is not always going to follow the other," Kaiser says. One station in Colorado, for example, showed a significant increase in the number of snowy days, but no increase in the total amount of snow. Areas with fewer snowy days, however, are more likely to have less snow overall.
Kaiser suspects that the trend holds true only for the "transitional" periods when winter is coming in, or going out.
Temperatures since 1948 have warmed by several degrees during the "beginning and end of the winter season," he says, so it's less likely to get cold enough to snow. Instead, people in the northeast are more likely to get rain, turning whatever snow they have into slush. During the main winter season, however, it remains cold enough for weeks at a time, so no significant drop in snowy days is likely to occur.
Like most meteorologists, Kaiser is reluctant to blame the change on global warming, at least at this stage, but he comes pretty close.
"It's consistent with the big picture that we've been observing (warmer temperatures) and we have a pretty high degree of confidence that we will be observing in the future," he says.