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SIOUX FALLS, S.D. (KELO) — As a storm front moved into South Dakota’s Sioux Falls on Tuesday, night seemed to fall early, as 3:00 p.m. saw dark skies cast into a deep and eerie shade of green.

But what causes this foreboding shade of green? According to, the sky normally appears blue due to the process of refraction. “During the day, particles in the air scatter more violet and blue light, and our eyes are more sensitive to blue.”

Meteorologist Scot Mundt with Nexstar’s KELO says that this same process produces a green sky, with the addition of one important ingredient.

“It’s caused by the sun’s rays being refracted by the hail (inside the storm) as the rays pass through the cloud,” Mundt explained. “It’s the same concept of how a rainbow forms. The sunlight is refracted as it passes through the raindrops, resulting in a rainbow.”

  • Wall Lake Area Storm as of 3:25 p.m. | Courtesy Paul Schiller

Mundt says that the reason that green light comes through more than others is due to the heavy rains within the storm stifling the other particles.

Many people also associate green skies with tornadoes, and Mundt says that’s not without reason. “The green sky is associated with tornadoes because the large hail typically falls on the northern side of the storm where a tornado may develop.”

Whether it results in hail or tornado, a green sky is a good indication that you should find some shelter.

“I’m an advocate for anytime there’s a warning people should seek shelter and not be ‘Lookie Lous,'” said Mundt.

The derecho that pounded South Dakota on Tuesday moved across a roughly 720-mile path stretching into northern Iowa and southern Minnesota, Mundt noted.

The storm earned a derecho designation because of long-lived wind and widespread impact. The National Weather Service’s definition of derecho is a “widespread, long-lived wind storm that is associated with a band of rapidly moving showers or thunderstorms.”.

This is the second derecho in South Dakota in three months. The state experienced a derecho on May 12.