Drought is typically thought of as a simple lack of rain and snow. But evaporative demand—a term describing the atmosphere’s capacity to pull moisture from the ground—is also a major factor. And the atmosphere over much of the U.S. has grown a lot thirstier over the past 40 years, a new study in the Journal of Hydrometeorology found.

Evaporative demand can be thought of as a “laundry-drying quotient,” says Nevada state climatologist Stephanie McAfee, who was not involved in the study.