Does a green sky mean a tornado is coming?

The sight of an eerie green sky before a fierce thunderstorm is sometimes perceived as a warning sign of an approaching tornado, but forecasters say that this long-standing myth might not be true.

Scientists aren’t completely sure why some intense thunderstorms turn the sky green, but research meteorologist Scott Bachmeier at University of Wisconsin-Madison says it could be linked to two factors: the color of the sky late in the day and the amount of rain in the storm.

The first factor is sunlight, particularly as evening approaches and the sun is low in the sky. Around sunset, sunlight interacts with Earth’s atmosphere in a way that can make the sky look orange or red.

The second factor is a tall thunderstorm with an abundance of rain. Bachmeier says that the way reddish light before sunset interacts with the large amount of rain in the storm could explain the ominous green clouds.

Massive thunderstorms with enough moisture to create a green sky are often severe enough to produce large hail and sometimes even tornadoes. However, seeing a green sky does not necessarily mean that a tornado is imminent. It is also not a reliable indicator of a storm’s intensity, since the change in color is likely linked to the time of day.

So, while a green sky doesn’t always mean a tornado is coming, it is a sign that you should take cover from an approaching thunderstorm.