In the 1996 movie Twister, Bill Paxton and Philip Seymour Hoffman watch an approaching storm. “Going green,” Paxton’s character says, to which Hoffman’s character replies, “Greenage.” They excitedly hurry into their chase vehicles to track the tornadic supercell. But why does a green sky sometimes foretell a tornado or severe thunderstorm?
Scientists have a number of theories that explain green skies before severe weather. Time of day is one of the most important ingredients. Here’s a closer look at some of the factors that may come into play.
First, let’s review why normally the sky is blue. The sky looks blue because, during the day, particles in the air scatter more violet and blue light, and our eyes are more sensitive to blue. The strongest thunderstorms tend to happen in the late afternoon, when the sun is descending toward the horizon. While the light under a towering thundercloud appears blue, the reddish tinge of approaching sunset illuminates blue objects to make them appear green. Scott Bachmeier of the University of Wisconsin-Madison says that green clouds:
only occur 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.
Another factor leading to green skies might be the water droplets themselves. Water is really good at reflecting blue light, but it can also reflect green light under just the right conditions, such as when a lowering sun’s rays shine on them. The right thickness of clouds, combined with the right diameter of water droplets and the right time of day may produce the perfect combination to turn the sky a greenish color.
It’s important to note that the sky doesn’t have to turn green for tornadoes to form. While a green sky is often an indicator of a severe storm that can produce tornadoes and damaging hail, a green sky does not guarantee severe weather, just as tornadoes can appear from a sky without a hint of green.
To sum up, the reason for green skies before a storm isn’t entirely known. But it is known that a greenish-yellow sky before a storm is common in some parts of the world, while totally absent in others.
Bottom line: If you see a storm approaching and the sky looks strangely green, it’s best to take cover. A severe storm that may produce tornadoes or hail may be on the way.
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