Researchers looked at power outages between 2018 and 2020, measuring outages by hour at the county level. The study covered 2,447 counties, or 77.9 percent of U.S. counties.

Overall, researchers found 231,174 outages that lasted an hour or more during the study period, and 17,484 that lasted eight or more hours. Of these, 62.1 percent occurred alongside “extreme weather/climate events.”

Lengthy outages are of particular concern because they can affect people who rely on electrically powered medical or mobility devices or need medications refrigerated. These “medically relevant” outages peaked during late spring and midsummer, researchers found.

Clusters of counties in Louisiana, Arkansas and northern Michigan had more outages and higher scores on the Social Vulnerability Index, a federal measure of factors such as poverty and overcrowded housing that can make it harder for communities to prepare for or respond to emergency events. The information could be used to improve disaster response and reduce the potential health effects of such outages, researchers write.

Severe weather increased the likelihood of lengthy outages, the analysis found, with long outages 10 times more common on days with multiple severe weather events and 51.6 percent more likely on days with simultaneous heavy rain, high heat and a tropical cyclone.

“We look at weather reports and decide whether or not to bring an umbrella or stay home,” said Joan Casey, an assistant professor in the University of Washington’s Department of Environmental and Occupational Health Sciences who led the research, in a news release. “But thinking about being prepared for an outage when one of these events is rolling through is a new element to consider.”