Green Sky Blankets South Dakota Before Derecho Swept Through State

The sky in South Dakota was an eerie shade of green on Tuesday, before a derecho brought winds nearing 100 MPH, per National Weather Service Sioux Falls.

A derecho is described by the National Oceanic and Atmospheric Administration as a “widespread, long-lived wind storm associated with a band of rapidly moving showers or thunderstorms.” The National Weather Service website says a storm receives a derecho classification “if the wind damage swath extends more than 240 miles (about 400 kilometers) and includes wind gusts of at least 58 mph (93 km/h) or greater along most of its length.”

South Dakotans have become used to derechos after experiencing two such occurrences recently, but the green sky was an unusual sight.

“I think it caught a lot of people’s attention because the sky did have that very unique green color to it,” said meteorologist Peter Rogers said, per the New York Times. “Because of the unique color that it did exhibit, I’m guessing that it will probably be a topic of discussion for quite a long time.”

It’s unclear why exactly the sky turns green ahead of a severe weather, but Frank Gallagher, a meteorologist who now works at the National Oceanic & Atmospheric Administration, told the Scientific American back in 2007 that the dominant wavelength of light in several severe thunderstorms is green. Researchers believe water droplets in the air that appear blue could be set against heavy red light, such as a sunset, and give off a green appearance.