A green sky is often synonymous with severe weather and a risk of tornadoes. However, this eerie color to the storm is actually caused by the angle of sunlight and how it interacts with the moisture in the cumulonimbus cloud.
For the sky to turn that color of green, you typically need the sun at a low angle, shining through a broader section of the atmosphere. This causes a phenomenon called Rayleigh Scattering, which reduces the violet and blue side of the color spectrum.
WATCH: Ask a Meteorologist: Why does the sky turn green during tornadoes?
Once sunlight hits the storm cloud, it interacts with raindrops and moisture particles, reducing the red end of the visible spectrum. Once that filtered light reaches our eyes, we are left with more of a greenish tint (the middle of the visible spectrum) from that part of the sky.
So why do we associate green with a dangerous thunderstorm? Well, the association is not unfounded. Most storms that produce tornadoes and large hail contain abundant water, which helps create this optical effect.
At the same time, intense storms often approach from the west late in the day, which is the optimal optical setup for seeing this as a storm bears down. Ultimately, though, this is a case of correlation and not causation.
The tornadoes and even large hail are not directly responsible for the green color. It just so happens the optical effect is present in many of those severe weather instances. For more on the green sky phenomenon, click here.
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