Why Does the Sky Turn Green Before a Tornado?

In The Wizard of Oz, Dorothy lives through a terrifying twister before finding herself in the green-hued Emerald City. But in real life, things are actually the opposite: in many instances, the sky turns green before a tornado.

Why does this happen? Do green skies really mean a tornado is coming? Keep reading to learn the basics behind this celestial phenomenon.

According to AccuWeather, there is no explicit connection between a green sky and tornadoes; however, it is common for a green sky to appear before a tornado. So, if the sky above you is suddenly as green as the Emerald City, you don’t necessarily need to prepare for disaster — but going inside and checking your local weather forecast is highly advised.

That said, a green sky often does occur before or during a tornado, thunderstorm, or other extreme weather event. Why is this? 

First, let’s go over why the sky is normally blue. As explained by NASA, as sunlight reaches planet Earth's atmosphere, the gases and particles in the atmosphere reflect the sunlight in every direction. Because blue light travels in smaller waves, blue light is reflected more than any other color, resulting in a blue sky during the day.

So what makes the sky go green when a tornado is imminent? 

Scott Bachmeier, a research meteorologist from the University of Wisconsin-Madison, explained in an article for the university that green clouds and skies only happen “if the cloud is very deep, which generally only occurs in thunderstorm clouds … Those are the kind of storms that may produce hail and tornadoes.” 

He added that a green sky also means a cloud is extremely tall — and because thunderclouds are very tall (in fact, they’re the tallest clouds), seeing a green sky often does mean a tornado or hail storm is in the works. 

This phenomenon also has to do with the combination of red sunsets and water droplets in the air that often occur during tornadoes, as per SciShow. (Most tornadoes occur around sundown hours.) Even though water droplets reflect blue light best, when tall storm clouds are present, the water droplets in the clouds are better able to reflect the green light into our eyes than they are able to reflect the warm colors of the sunset — making the sky appear green.

Craig Bohren of Pennsylvania State University explained to AccuWeather that as the sun begins to set, the sky changes from blue to warmer colors like red and orange. However, “When this setting light is transmitted by a massively thick cloud composed of water droplets and ice particles, the results are a green sky,” Bohren explained.

And while green skies are often associated with tornadoes, many believe that their direct association is an overrated myth — green skies are just as likely to occur alongside thunderstorms or other severe weather events.

"Green skies are associated with severe weather," Bohren told Scientific American. "In areas where tornadoes are common, they are said to be the cause of green storms. Or you will be told, often with considerable vehemence, that hail causes the greenness. Both explanations are easily refuted by observations."

There you have it — while incoming thunderstorms can happen alongside green sky, the two events do not always occur in tandem. Plus, other weather events are also associated with green skies. So if you suddenly notice a green tint to the sky above you, rough weather might be ahead. But at least the Wicked Witch of the West probably isn't coming for you — nor your little dog.