Dreaming Of A White Christmas?
If you're in Minnesota, chances are better than not that this Christmas will be a white Christmas. In our warming climate, though, the chances of a while Christmas are going down across the U.S. as a whole.
Defined as 1" or more of snow on the ground at 7am CST on December 25th, a "white Christmas" occurs between 30% and 90% of the time, depending on where in Minnesota you live. Here in the Twin Cities of Minneapolis and St. Paul, 3 out of every 4 Christmases are white.
While overall weather pattern that happens to set up in the days before Christmas -- quiet, or stormy -- has a direct impact on whether there's snow cover, it's also true that temperatures have to be cold enough for snow to fall and stick around. It stands to reason a warmer climate would create less opportunity for snow to form, and a greater likelihood it will melt away quickly, even when it does.
Here in the Twin Cities, and across North America as a whole, average snow cover has declined noticeably since 1973 (when consistent and accurate snow cover statistics became available). Scientists at Climate Central attribute this to human-caused climate warming worldwide, but say the relationship is complex.
“One reason is because an increasing percentage of winter precipitation is falling as rain instead of snow in many locations. A Climate Central report found that between sea level and 5,000 feet in elevation across the Western U.S., a smaller percentage of winter precipitation is falling as snow.
However, rising temperatures can cause some individual storms to produce more snow, where temperatures are still well enough below freezing. That’s because for every 1°F rise in temperature, the atmosphere can hold 4 percent more water vapor. In turn, more water is available to fall as snow or rain.”
Curious about the chance of a white Christmas in other cities around the country? This interactive map from NOAA shows you the historical probability that 1" or more of snow will be on the ground on Christmas Day at 7am local time.