It’s true the sky can turn green before a tornado. As a Nebraska native, I’ve witnessed the phenomenon firsthand numerous times. While thunderstorm clouds may appear green or yellow before a tornado, they may also turn these colors before a hail storm. It’s also possible for the sky to turn green without producing a tornado or hail, but the thunderstorms are almost always severe.
While the green sky is well-documented, the reason for the color isn’t as well understood. Partly, this is because there isn’t a lot of funding money available for the research. Also, there are multiple reasons for the green or yellow color, depending on the time of day and the characteristics of the storm.
De-Bunked Explanation for the Green Sky
Some explanations for the green sky before a tornado or hail have been de-bunked as false:
- Optical Illusion: Meteorologists have measured the wavelength of light from clouds using spectrophotometers and verified the light is actually green.
- Reflection of the Ground: Scientists have verified the sky color is not just a reflection of green fields, because the spectra don’t match up and because the color has been observed over red and brown soil.
Green Clouds and Water Vapor
On theory that holds water, so to speak, is that certain thunderheads filter wavelengths of light, leaving only green. Computer models verify the thickness of the cloud combined with the diameter of the water droplets can produce a green color. A powerful thunderstorm has the right size of clouds, optimal amount of water, and may produce a tornado.
Sunrise and Sunset Green Skies
Some areas are more prone to tornadoes than others, such as parts of Australia and the Great Plains of the United States. In these regions, tornadoes form more often during certain months and certain times of the day, with most storms forming late in the afternoon. So, the most common cause of green skies is the interaction between the golden to reddish light of sunset and the blue of water in clouds.
Here’s how it works. During the day, the sky appears blue because blue light bounces off molecules in air (is reflected) and is scattered more effectively than other parts of the spectrum. As the sun rises or sets, the light illuminates the sky at an angle. Much more scattering occurs, bouncing most of the blue light away from the horizon. While sunrise and sunset look mainly red and gold, green light is still present in the mix. Normally you don’t see the green, but a column of water droplets reflects blue and green light better than it reflects red and orange. While a strong thunderstorm late in the day has the best chance for producing a green sky, the right combination of cloud thickness and light can still make a green sky at midday.
Hail Refracting Light
Strong thunderstorms have an updraft that draws warm, moist air upwards and suspends water droplets. As water droplets fall and cool, they can freeze, accumulate more droplets, and become hail. Hail has different optical properties from liquid water droplets. If the angle of the light is just right, falling hail can refract light and appear green. In this situation, the clouds appear normal, but the sky between the base of the clouds and the ground is green.
Lightning color can make the sky appear green. Tornadoes typically have a lot of lightning, so they may be illuminated by their light. Under a clear sky, lightning is white. However, it can appear yellow, orange, or red, depending on the amount of dust in the air. Yellow lightning against blue clouds can make them appear green.
Tornadoes aren’t always the same color as their clouds. The direction of lighting can color the tornado, even if the clouds producing it appear blue. Pink, orange, and yellow tornadoes have all been observed. Tornadoes over red soil can be red, while those traveling over snow may be white. Some tornadoes are invisible except for the debris at their base.
Tornadoes Without Green Skies
While a green sky is a clear warning of a dangerous storm, tornadoes and hail often come from normal blue or gray skies. The sky is more likely to appear normal when the storm occurs early in the day.
- Bluestein, Howard B. (2006). Tornado Alley: Monster Storms of the Great Plains. Oxford University Press. ISBN: 978-0195307115
- Bohren, C. F.; Fraser, A. B. (1993). “Green thunderstorms.” Bull. Amer. Meteor. Soc., 74, 2185–2193.
- Edwards, Roger (2009). “Public Domain Tornado Images“. National Weather Service. National Oceanic and Atmospheric Administration.
- Gallagher, Frank W. (1999). “Distant Green Thunderstorms– Fraser’s Theory Revisited.” Journal of Applied Meteorology.
- McCartney, E. J. (1976). Optics of the Atmosphere: Scattering by
- Molecules and Particles. John Wiley and Sons.
- Snow, John (2020). “Tornado.” Encyclopædia Britannica.