A white Christmas is going from reality to more of a dream for chunks of the United States.

Analysis of 40 years worth of United States snow measurements shows less of the country now has snow on the ground on Christmas than it did in the 1980s. It’s especially noticeable in a large swath in the east-west middle belt of the country.

But the data is complex and some scientists caution against drawing conclusions about trends and the role of global warming. Places such as Iowa, much of the Rockies and eastern Washington have lower chances for a white Christmas.