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.
"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.