The intensity of rain is rising in most of the U.S., mainly in eastern states, study of two 30-year periods concludes

It's raining harder in the U.S., reports a study published Tuesday by researchers at Northwestern University in the peer-reviewed journal Geophysical Research Letters. "The research offers confirmation of what atmospheric scientists have been warning of for years: a warmer world is, on balance, a wetter world. And as global temperatures continue to rise, an uptick in precipitation extremes is expected," meteorologist Matthew Cappucci writes for The Washington Post. 

A man launches a boat during late July's severe flooding in Eastern
Kentucky. (Photo by Ryan C. Hermens, Lexington Herald-Leader)

The study did not rely on forward-looking projection models, as many recent studies have, but used historical data to compare rainfall over two periods, 1951-1980 and 1991-2020, to see what changed, Cappucci reports. In the Eastern U.S. the researchers saw a 4.5% to 5.7% increase in average daily rainfall on the days that it rained. That doesn't mean that it rained on more days or that there was more rain overall, but there "may be places where precipitation intensity is increasing but frequency is decreasing. We might not know if there’s an overall increase or decrease. That’s one thing that we’re working on," said Ryan Harp, the study's lead author. 

Data for the severely drought-stricken Western U.S. offered "mixed signals," Cappucci writes. The overall trend didn't hold true there, especially the Pacific Northwest. Citing Harp, Cappucci writes that "changes in the overall placement of weather systems are 'suppressing' any tendency for heavier precipitation in the West."

Cappucci notes that the findings of the study are consistent with a basic tenet of atmospheric physics: "For every degree Fahrenheit that air temperature rises, the atmosphere can hold 4 percent more water; this is known as the Clausius-Clapeyron relationship. Where storm clouds develop and the atmosphere is sufficiently moist, it means a warmer climate will support more intense rainfall." It could help explain why the U.S. has seen five 1,000-year rain events in a five-week span this year.