SIOUX FALLS, S.D. (KELO) — As the storm front moved into Sioux Falls on July 5, night seemed to fall early, as 3:00 p.m. saw dark skies cast into a deep and eerie shade of green.

Green sky over Sioux Falls as storm approached

But what causes this 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.”

KELOLAND Meteorologist Scot Mundt 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.”

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.

  • Wall Lake Area Storm as of 3:25 p.m. | Courtesy Paul Schiller
  • Storm coming into downtown Sioux Falls

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.