09-20-2003 - Earth

sagar: Author=’Deborah Byrd’ program=’3911′ video=”Photo courtesy of The University of Massachussetts. DB: This is Earth and Sky, with a listener’s question: “Why does the sky turn a strange color before a tornado? ”

JB: No one knows for sure – but there are a couple of theories about that weird greenish-yellow color you see before a storm. Storm clouds themselves may help make the color green – or the storm may just act to provide a dark backdrop, so that you can see the color.

DB: Here’s one idea. You know how mountains in the distance look a deepening blue. That blue color stems from the scattering of light by air molecules – it’s the same reason the sky looks blue. In one theory on green thunderstorms, a golden-reddish light from the sun low in the sky – and a natural bluing effect of the air – combine to create the green sky.

JB: The other theory about green skies during a storm has to do with the fact that water is intrinsically blue. Sometimes you can see the bluish tint of water in a white bathtub. It could be that the storm clouds provide the color blue – which again is illuminated by the golden light of a low sun to create the color green. So the reason for green skies before a storm isn’t entirely known. But it is known that a greenish-yellow sky before a storm is common in some parts of the country – while totally absent in others.

DB: For today, that’s our show. Special thanks to the “National Oceanic and Atmospheric Administration”:http://www.noaa.gov and to the “National Fish and Wildlife Foundation”:http://www.nfwf.org/. We’re Block and Byrd for Earth and Sky.

Written by EarthSky