A white Christmas seems to be slowly morphing from a reliable reality to a dream of snowy holidays past for large swaths of the United States in recent decades.

Analysis of 40 years of Dec. 25 U.S. snow measurements shows that less of the country now has snow for Christmas than in the 1980s.

That’s especially true in a belt across the nation’s midsection — from Baltimore to Denver and a few hundred miles farther north. And snow that falls doesn’t measure up to past depths.

Scientists say the decline in the number of white Christmases is relatively small and caution about drawing conclusions. But it’s noticeable and matters mightily to some people like George Holland.

The retired Dubuque, Iowa, educator known for his front yard nativity scenes said snow on Christmas is supposed to be part of the holiday: “The one that makes my heart warm is after going to midnight Mass and coming outside and it’s snowing.”

But the weather in Dubuque hasn’t cooperated in recent years. “We don’t have white Christmas,’’ said boutique owner Bill Kaesbauer. “We haven’t had any in years.”

The last one was in 2017 in Dubuque, which weather records show used to have white Christmases nearly two out of three years.

The average December temperature in the continental U.S. was a tad below freezing from 1981 to 1990, federal weather records show. And from 2011 to 2020, it was up to an average slightly above 35 degrees, considerably above the freezing mark.

But what did that warming trend, natural weather variability and a western megadrought mean to white Christmases?

From 1981 to 1990, on average, almost 47% of the country had snow on the ground Christmas Day, with an average depth of 3.5 inches, according to an analysis of ground observation data by the University of Arizona for The Associated Press. From 2011 to 2020, Christmas snow cover was down to 38%, with an average depth of 2.7 inches.

The change was particularly pronounced in a swath from about the Mason-Dixon line to just north of Detroit, Chicago, and Nebraska. The Christmas snow cover average there went from nearly 55% in the 1980s to slightly above 41% now, the Arizona data shows. Average snow depth fell from 3.5 inches to 2.4 inches.

The numbers are small enough that it’s difficult to tell whether this is a meaningful trend and, if so, whether climate change or natural weather variability is the cause, said University of Arizona atmospheric scientist Xubin Zeng, who ran the data. Still Zeng, who has published studies on decreasing snowpack in the western U.S. being connected to climate change, said the downward slide of white Christmases is consistent with global warming.

In 20 to 30 years “with climate warming, the prospects of a white Christmas in many parts of the U.S.A. will be slim indeed,” said Mark Serreze, director of the National Snow and Ice Data Center in Boulder, Colo.