The sky in Sioux Falls, South Dakota, darkened and turned an eerie shade of green on Tuesday as a derecho swept through the region.
A derecho is a cluster of thunderstorms and meteorologists will only classify a windstorm as such if the wind damage will cover a distance of at least 400 miles.
Onlookers and weather experts shared images of the phenomenon before the storm hit.
According to local Fox Weather meteorologist Heather Brinkmann, the green color is created from light refraction within the thunderstorms and indicates that the storm will be capable of producing large hailstones.
— Heather Brinkmann (@WeatherHx) July 5, 2022
It takes a “tremendous amount of water content within the cloud to achieve this color, which usually means a substantial amount of ice (large hail) has to be present,” National Weather Service meteorologist Cory Martin shared in a graphic.
Pics of the green sky from my family in Sioux Falls have been incredible! Had to dig up this fun green cloud infographic I made back in the day. Hail reports not too large at the moment, but storms are very moisture loaded in this warm, tropical environment. #SDwx#IAwxpic.twitter.com/S39JLSRfEu
— Cory Martin (@cory_martin) July 5, 2022
— jaden 🥞 🍦 (@jkarmill) July 5, 2022
The color of Sioux Falls’s sky rn is unreal. pic.twitter.com/IuS6bB1PSk
— Carl Jones (@Wx_Jones) July 5, 2022
Indeed, there were later reports of large golf ball-sized hail in parts of the region.
This article originally appeared on HuffPost and has been updated.