Why do skies turn green before a tornado

Meredith Knight is a freelance science and health writer based in Austin, Texas. Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue.

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Sign Up. Support science journalism. Knowledge awaits. See Subscription Options Already a subscriber? Recent research suggests that a tornado's characteristic spirals build from the ground up , the inverse of the process meteorologists thought to be the case for decades.

If researchers are able identify how twisters form and land on concrete, early signs that one is coming, alerts to anyone who might be in the path can go out sooner. Right now, the average gap between when meteorologists issue a warning and when the tornado hits is 14 minutes, says Kelsey Ellis, a hazard climatologist at the University of Tennessee Knoxville.

That being said, a team of researchers probed the green-sky mystery briefly in the s. The researchers found that storms in Oklahoma and Texas sometimes looked green. When the crew aimed a device that measured wavelengths at the skies , the technology picked up on rays in the green portion of the visible spectrum. VORTEX researchers hypothesized that the eerie hue comes from sunlight beaming through water-filled storm clouds.

In the Midwest in particular, tornadoes tend to form later in the day, when the setting sun casts yellow, orange and reddish rays over the sky. As the sunlight comes out the other side of the brewing storm, the interference of the blue water makes the light green.

Only those in the direct path of the sunlight as it cuts through the clouds would see the result, which is why people in one area but not in another might see the sickly color during a storm, Krocak says. Even professional storm chasers can struggle to connect what they see to radar results and gauge risk, Ellis says. Ideally, people in the path of a tornado shouldn't waste time staring at the sky.

So researchers are fine-tuning how meteorologists should communicate risk to get people acting responsibly, fast. But that doesn't necessarily mean a twister is coming—it could just mean a lot of rain is in the forecast. Either way, heading inside is probably a good idea. For the full details on how water and light conspire to turn the sky green before a storm, check out the SciShow video below.