On 10 August 2020, a derecho roared through parts of the central United States, covering an area close to 90,000 square miles (240,000 square kilometers) and affecting nearly 20 million people in its path from South Dakota to Ohio. The strongest wind gusts were captured at 140 miles per hour (225 kilometers per hour), with the most destruction reported in central and eastern Iowa and parts of Illinois. With $11 billion in damages, the thunderstorm was the costliest in modern U.S. history.
Putting it into slightly different storm terminology, the 2020 derecho was like a category 4 hurricane in terms of wind speed, said Andreas Prein, a scientist at the National Center for Atmospheric Research.
Some scientists suggest that extreme weather may be intensifying as the climate changes. Heavy precipitation increases in a warming atmosphere, but the connection between relatively rare wind events and climate change isn’t yet fully understood. Thunderstorm wind gusts, for instance, are more localized than rainfall from severe storms; this makes the gusts harder to observe or model.
In a recent study published in Nature Climate Change, Prein examined trends associated with thunderstorm wind events in the United States over the past 40 years. His findings indicate a connection between intensifying thunderstorm wind gusts and a warming climate.
A Mighty Wind
Thunderstorms can be challenging to study and forecast because they often affect only a small geographical area, unlike a hurricane or winter storm.
Derechos and other damaging straight-line winds called downdrafts are sometimes likened to an unfurled tornado; both are associated with thunderstorms, but unlike a tornado, derechos and downdrafts move in one direction rather than rotating. “Derechos form when an organized line or group of thunderstorms produce a persistent path of dangerous and damaging straight-line winds,” said Hutch Johnson, a meteorologist based in Fargo, N.D., who was not part of the new study. “They can travel for hundreds of miles and produce areas of serious damage.”
Severe thunderstorms are defined as storms capable of straight-line gusts of more than 58 miles per hour (93 kilometers per hour). “Wind that strong can snap and flatten trees,” said Johnson.
Thunderstorms can be challenging to study and forecast because they often affect only a small geographical area, unlike a hurricane or winter storm, said Johnson. Climate models are often too coarse, meaning the grid spacing is too big and broad to drill into the specifics about the intensity of one thunderstorm cell’s behavior. Because these sorts of wind events are often rare and short-lived, relying on observational data isn’t enough, either.
Forty Years of Straight-Line Winds
Standard climate models feature grid spacing of hundreds of kilometers, said Prein, way too large for investigating storms on a smaller scale. In his study, Prein debuted higher-resolution computer modeling, reducing the grid spacing to only 4 kilometers. Prein’s model homed in on 109,387 data points across a map of the U.S. Midwest and revealed how the area affected by strong straight-line winds increased by 4.8% between 1980 and 2022.
Prein combined the data gleaned from the high-resolution models and cross-checked them with information recorded at 95 weather stations in the Midwest. The model’s finding largely matched recorded thunderstorm wind gusts (including the 2020 derecho), correlating observations with the model’s output.
“The main innovation of this exciting study is that Prein combines observations and a very high resolution model to extract a clear climate change signal that so far was not known and explains the key physical mechanisms for these changes,” said Erich Fischer, a climate scientist at ETH Zürich who was not involved in the study.
Straight-line winds occur when rain evaporates aloft, cooling the air, and then that heavy, cold air rushes down to the ground. By increasing the temperature, which is occurring with climate change, evaporative cooling during a thunderstorm increases as well, Prein said.
“You can store more moisture in a warmer atmosphere, so you can evaporate more. This means that the temperature difference between the surrounding air and the air that falls out of the thunderstorm is getting bigger, which causes a stronger downdraft and a stronger wind at the surface,” he explained.
This study is an important first step toward understanding the connection between thunderstorm wind gusts and climate change, said Fischer. “The findings will need to be confirmed with other models,” he added. “Ideally, simulations at even higher spatial resolution would help to study these small-scale events in more detail.”
Preparing for the Next 40 Years
For the millions of people who live in areas with increasing chances of a straight-line wind event, building resilience and understanding is key to protecting lives and property as the climate continues to warm.
Critical infrastructure, including hospitals, power grids, industrial buildings, and even public transportation, should be reinforced to withstand strong winds, said Fischer.
“As far as I can tell from the historic record, we see a clear increase [in straight-line wind events], and the increase is not slowing down.”
In addition, he added, “it will be interesting to see whether probabilistic warning systems could be developed to issue at least short-term warnings, to close exposed roads and avoid traffic accidents.”
More detailed weather forecasts, too, could help residents know when a severe thunderstorm alert includes the risk of a derecho or another severe straight-line wind event.
After examining data from decades in the past, Prein now has plans to look toward the straight-line winds of the future—his newest study involves running the models from 2021 through the 2060s. “As far as I can tell from the historic record,” he said, “we see a clear increase [in straight-line wind events], and the increase is not slowing down.”
—Rebecca Owen (@beccapox), Science Writer