Here's why the sky literally turned green during a thunderstorm in South Dakota Tuesday

SOUTH DAKOTA, USA — You might've seen the dramatic green-colored skies that created stunning visuals over Sioux Falls, South Dakota on Tuesday. If not, check out the pictures below.

You might be wondering how a sky could turn green, and what, if anything, that means about the possible strength of a storm.

The answer to both of those questions comes down to one simple scientific term: Refraction.

Refraction, simply put, is the bending or deflection of light waves based on the speed of the light wave. It's what creates rainbows, different colored sunsets and water mirages in the desert.

Water and ice are dense objects. Air is light. So when light travels between air and water, the speed of the light wave dramatically adjusts. If you have a lot of ice (hail) or water (heavy rain) in the sky, therefore, it's prone to getting refracted.

Tuesday, a long, powerful line of severe storms approached Sioux Falls during the middle of the day. Since we're just past the summer solstice and Tuesday's storms hit there around 3:30 p.m., it meant the sun angle was extremely high as the storms approached.

If you couple the very high sun angle with a lot of heavy rain and hail that accompanied the extremely strong storms, you had prime conditions for maximum light wave bending (refraction).

If you see a green or blue-tinted sky like the ones over South Dakota, that's usually an indicator that there's a lot of something in the sky. It's likely a sign there's a lot of water or ice, and often the latter. 

In other words, it's a pretty good bet that if you see a green or blue sky, a really nasty storm is close by.

Here in Colorado, we can get these types of blue or green-colored clouds before or during storms. Normally, ours come from big hail that's suspended up in clouds. If there's a greenish tint to the sky over Colorado, it's often (though not always) an indicator that big hail could be just around the corner.

So if you ever see a sky like the ones Sioux Falls saw on Tuesday, look out: Nasty weather is probably on the way.

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