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The skies above Sioux Falls, South Dakota, turned a strange and slightly apocalyptic shade of green on Tuesday, as a powerful storm swept through the state.
Images of the surreal sky were widely posted on social media.
So, in the words of Kent Brockman from The Simpsons, is it time to crack each other's heads open and feast on the gooey bits inside? Well, no. Or at least not because of the green sky, which is a perfectly natural – if rare – phenomenon.
The effect – though undoubtedly eerie – is caused by the light being scattered by ice.
"Water/ice particles in storms with substantial depth and water content will primarily scatter blue light," NWS meteorologist Cory Martin explained on Twitter.
"When the reddish light scattered by the atmosphere illuminates the blue water/ice droplets in the cloud they will appear to glow green. 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."
Just as red sky at night means shepherd's delight, and red sky in the morning is a shepherd's warning, a green sky means "look out for big chunks of ice falling at your head." While not the best rhyme, it is still good advice.
"This phenomenon is typically a visual warning sign that the thunderstorm is capable of producing very large hail," Martin explained.
