You see those green, billowing storm clouds over there? Those are bad. Not because you’ll get whisked away to some annoying musical world—no, because those clouds mean that storm is particularly nasty and dangerous. It might even mean a tornado is approaching.
Green clouds have long been considered a signal of a coming tornado or hail storm by the people living in Tornado Alley, a large are of the U.S. that spans northern Texas, Oklahoma, Kansas, Nebraska, and beyond. Traditional folk wisdom says that the ominous color of the clouds comes from all the frogs and grasshoppers the tornadoes sucked up into the sky. How ribbiting! (Crickets chirp.) But don’t worry, frog and grasshopper lovers, that’s obviously not what’s going on with these scary water vapor behemoths.
So why the green? Youtuber and stormchaser Pecos Hank explains that it all comes down to the color spectrum, and the way objects absorb and reflect sunlight:
Some of the biggest thunderstorms can generate areas of concentrated heavy rain and hail that look like waterfalls. Storm chasers call this the “core.” Sometimes storm clouds part giving you a glimpse into the core aloft. If you’re underneath the storm base, around midday, the core is often blueish or a gorgeous turquoise. But in the late afternoon and evening when most storm activity occurs, angled golden and reddish sunset light might mix with the blue precipitation core and the net effect is a greenish color. Yellow and blue makes green!
William Beasley, professor of meteorology at the University of Oklahoma, agrees. He explains to Scientific American that it all comes down to the moisture particles found in these afternoon thunderstorm clouds. They absorb red light and make the scattered light appear blue. When they’re set against an environment heavy red light, like a low sun during “golden hour” or a sunset, the net effect is a sort of greenish color. Here’s the thing, though: according to physicist Craig Bohren at Pennsylvania State University, green storm clouds don’t necessarily mean a tornado or hail storm is coming. Basically, a hail storm or tornado can happen with or without green clouds. That’s why it’s important to be prepared for such events and not only rely on what you can see.
That said, green clouds are a fairly decent predictor of severe thunderstorms in general. Green clouds are often tall clouds, says Scott Bachmeier, a meteorologist at the Cooperative Institute for Meteorological Satellite Studies at UW–Madison, and tall clouds are a major ingredient for potential hail storms and tornados. So, while seeing green doesn’t mean a tornado is coming, it does mean that a tornado could be coming. No matter what, you’re in for some severe weather, friend. If you’d like to avoid heavy rain, high winds, and lightning, green skies are still a good warning sign to pack it up and get out of dodge. If you see some green clouds coming your way in the afternoon, it’s time to leave the area, or at least go inside where you can easily follow your emergency plan if need be. And if you don’t have an emergency plan yet, you should make one.