Why Skies Turn Green in Thunderstorms

Jon Erdman
Published: April 27,2016

You may have seen an approaching thunderstorm turn parts of the sky a rather creepy shade of green or bluish green.
One example of a green sky was seen in the western suburbs of St. Louis on Tuesday as severe weather rolled through the area. The Weather Channel Facebook friend Nathan Pflantz shared with us this astonishing photo from Chesterfield, Missouri.

A green sky associated with an approaching line of thunderstorms in Chesterfield, Missouri, on April 26,2016.
(Nathan Pflantz/Facebook)

Given the rarity, one can understand the increased sense of foreboding, apart from the already dark sky and, usually, claps of thunder.
But is this a green sky a sign of an impending tornado or huge hail?

It's Not Easy Being Green

As it turns out, a greenish sky is not necessarily a sign of either, despite some existing folklore to the contrary.
However, the exact cause of this is still subject to debate today.
A 1993 study from Penn State University contends that there simply needs to be a strong thunderstorm with a large volume of precipitation and the right alignment of the sun and thunderstorm to turn the sky green. Researchers calculated hail's contribution to the green color was actually small.
The study's authors, Dr. Craig Bohren and Dr. Alistair Fraser, offered two theories.
Bohren submitted that sunlight behind the thunderstorm is attenuated and scattered by the rain and/or hail shaft to yield a bluish hue. If this thunderstorm occurs around sunrise or sunset, when the sun takes on a more red/orange/yellow look thanks to a longer trip of the sun's rays through the atmosphere, that thunderstorm could instead look more green.
Fraser, on the other hand, suggests sunlight ahead of the storm is the key. The sun shining on a thunderstorm with the combination of sunlight scattering in clear air - typically yielding blue sky - and the red/orange/yellow colors of a sun low on the horizon can lead to the greenish sky, Fraser wrote

Above: An approaching hailstorm in Wylie, Texas on April 12, 2016, takes on a bluish-green hue.
Another study three years later headed by Dr. Frank Gallagher at the University of Oklahoma and co-authored with Bohren analyzed data from a color photospectrometer and also found the absorption of sunlight from a setting or rising sun by rain and hail could lead to a green sky.
Gallagher claimed that hail cannot possibly produce the green sky, but the size of the drops dictate the exact shade of green. Namely, smaller (larger) drops lead to a blue-green (yellow-green) sky.
In fact, it's possible, according to Gallagher, that all thunderstorms have some greenish tint at some time in their life cycle, but that they're not often seen at the time.
There is no correlation between green skies and tornadoes, but The Weather Channel severe weather expert Dr. Greg Forbes has had two prior experiences with "green sky tornadoes".
"One of the green thunderstorms I've seen was tornadic, striking my hometown of Latrobe, Pennsylvania, in 1976," wrote Forbes in an internal memo.
"I saw one from inside the cloud on an aircraft in 1977 that had a funnel cloud. I've seen others that had a turquoise tint that didn't have reported severe weather."
We would love to see your photos of green skies. Share them with us on Facebook, on Twitter, or upload them to us at weather.com/photos.

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