Fact or fiction? A green sky means a tornado is coming

By Amanda Mitchell, AccuWeather staff writer

Is it true that a green sky means a tornado will be touching down any minute or is that just a long-standing myth?

While it is not uncommon for severe weather to accompany a rare green sky, there is also no direct correlation between the two.

The spectacle is not highly studied, although Dr. Craig Bohren and Dr. Alistair Fraser, meteorology professors at Penn State University, are among the few that have analyzed the phenomenon.

The meteorologists produced a joint study defining why the sky illuminates green. Neither of the published studies indicate that a tornado or hail manufacture a green sky.

Instead large, thick thunderstorm clouds directly impact the way we perceive the color of the sky explaining why the eye perceives it as green.

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Bohren explains that as the sun falls lower in the sky, the spectrum of direct sunlight is shifted from blue toward a perceived red, yellow and orange.

“When this setting light is transmitted by a massively thick cloud composed of water droplets and ice particles, the results are a green sky,” said Bohren.

The professor compared the phenomenon to putting a glass of water with a drop of yellow food coloring behind a glass of water with blue food coloring to produce the same tint of green that the light in the sky transmits.

Such large thunderstorm clouds have the potential to produce severe weather such as tornadoes, large hail, frequent lighting and flash flooding, but do not guarantee any of these potential outcomes.

Even in areas around the world where severe weather is common, a green sky is still very uncommon, making it difficult to generate data for continual research.