Back when Bing Crosby first sang Irving Berlin’s “White Christmas,” his dreams came true about 33 percent of the time, which actually is a very respectable batting average, dreams-wise. Our own experience with dream realization has been considerably less satisfying. We are not bitcoin billionaires. We haven’t won an Oscar, Congressional Medal of Honor, Heisman Trophy, or the local Rotary Club drawing for a free car wash. The world hasn’t perfected a grilled-cheese ATM, nor do we own a pygmy zebra named Octavius. Suffice it to say, then, that one out of three seems pretty darn good to us.
That 33 percent comes courtesy of Ethan Gutmann, a hydrologist at the National Center for Atmospheric Research in ever-atmospheric Boulder, Colorado. In assisting us, Gutmann had his work cut out for him, as weather data is collected at thousands of stations scattered across the United States, not all of which measure the same things. In short, old weather records are an imperfect, fragmentary morass, and a bit of a pain to sift through and compile.
Undaunted, Gutmann was kind enough to review data from about 1,000 weather stations—skipping places like Miami and San Diego—to analyze snow depth from December 20 to 31 (our “white-Christmas window”) between the 1930s and 1950s, then compare it to the same span from 1996 to 2016. Putting aside the customary array of caveats, qualifications, and nerdy-scientist niggles, what he discovered, more or less, was that the chance of a white Christmas nowadays is about 10 percent lower than it was back in Bing’s prime. Why’s that? “Almost certainly anthropogenic global warming,” Gutmann says. (Those preparing for the SAT should know that anthropogenic means “caused by man.”)
Really, though? Global warming? What about all those monster storms in recent years—the ones hyperventilating news channels refer to by such scientific names as “Snowmageddon!” “Snowpocalypse!” and “Snowfrickinwayyouregoingtoworktomorrow”? Turns out that as the winter months warm up, we might actually see more snow. “The largest snowfalls occur when the temperatures are just below freezing, around 28 to 32 degrees Fahrenheit,” says Gutmann’s NCAR colleague Kevin Trenberth. As temperatures drop, the atmosphere holds less water, meaning that, ironically, it can be “too cold to snow.” Warmer air means more moisture, which means more snow per storm—generally those heavy flakes that drag down tree branches and power lines, adding to the sense of wrack and ruin.
If you’ve followed this far, you not only deserve to have one of your dreams come true, you’re also probably wondering how a white Christmas is 10 percent less likely when big snowstorms are more likely. Great question. While rising temperatures produce more snowfall per storm, they also cause that snow to melt much faster than it might have in the 1940s. So while you may enjoy a white Pearl Harbor Day (December 7) or a white International Migrants Day (December 18)—and, hey, who doesn’t?—that doesn’t mean your Christmas will be snowy. We may as well sing along with Bing and hope for the best (while reducing our carbon footprints, of course).