MOLINE, Ill. — Tuesday night, July 5, a derecho made its way through Iowa. Before the derecho reached the Quad Cities area, it traveled through South Dakota. There, it created a green apocalyptic-looking sky that went viral on social media.
There is a scientific reason why the sky turns green. It is not a sign that the end of the world is coming. The reason why the sky turns green is due to the scattering of light.
Sunlight that reaches the earth’s atmosphere is scattered in all directions by gases and particles in the air. During the day, we see a blue sky as a result of this scattering. The reason why is that blue light is scattered more than other colors. Blue light travels a shorter distance and has smaller wavelengths. The sun is closer to us.
During sunsets, we see yellow and red lights for the opposite reason. Red/yellow is scattered the least, travels a long distance and has longer wavelengths. Sunlight during sunrise/sunset is passing through more of the atmosphere so red and yellow pass through. The sun is furthest from us.
When the derecho went through South Dakota, it occurred close to sunset when the sun was the furthest from the earth, so the sky already appeared more red/yellow. The light in a cloud is blue due to scattering by water particles and ice particles within the cloud. The blue objects are illuminated with red light and the sky appears green.
For a green sky to occur, there must be a lot of water within the cloud. This phenomenon occurs usually with deep clouds, these clouds have a lot of moisture within them. This indicates that there is hail present often large hail. Golf ball to tennis size hail was reported on July 5 in South Dakota.
If the sky does turn green, you can expect strong-to-severe weather in the form of either large hail or tornadoes.
There is a misconception that a green sky means a tornado will appear. The clouds present during a green sky can produce tornadoes, but there is no guarantee. There have been times when there is a green sky with no tornado, and there are also times when a tornado does occur.