text: why are the clouds before a tornado green?   while, dangerous thunderstorms that drop hail and spin up tornadoes seem to be routinely observed as turning green ("going green" in the movie twister), scientists are trying to find out what exactly is going on.   they have confirmed that some thunderstorms do turn green, and have various theories about why.   one possibility is that thick, thunderstorm clouds, with their water vapor, rain and hail, scatter sunlight in such a way as to give the clouds a blue hue.   when the sun is low in the sky, such as before sunset when most severe storms occur, the reddening light of the sun when shining through earth's atmosphere makes the bluish cloud tint turn green.   or, maybe storm clouds act as a kind of canvas upon which low sunlight, scattered by particles in the air, paints a greenish color.   both of these could be correct, and they may play in concert with each other to create the observed green tint.   in the spring of 1995, a university of oklahoma graduate student used a special device called a spectrophotometer to measure the wavelength of light emitted from thunderstorm clouds during project vortex, a major study of tornadoes. (related: u. of okla.: green thunderstorm presentation).  his research confirmed green, greenish-yellow, and greenish-blue colors of the clouds, while ruling out lush farmland as the source of the green color.   but the researcher concluded that the actual green color observed may be different for different people. in other words, what may look strikingly green to one person, might be more greenish-blue or greenish-yellow to someone else. maybe not even green at all.   all that's known for sure is that green thunderstorms do exist. it's not known whether all green thunderstorms are severe, nor whether all severe thunderstorms turn green.  the best advice: don't wait to see green clouds before seeking shelter from severe thunderstorms.