Why does the sky sometimes turn green around thunderstorms?

Answer 1

This is usually seen in the hail shafts of strong thunderstorms. Light relfects of the hail (and sometimes heavy rain), in a way to produce a green tint to the sky.

Examples of green hail shaft:
http://www.mthurricane.com/VCP-Hail_Shaft.jpg
http://www.australiasevereweather.com/photography/photos/1998/1113dc06.jpg

Answer 2

Why does the sky sometimes turn green during thunderstorms, especially hailstorms and tornadic storms?

While, dangerous thunderstorms that drop hail and spin up tornadoes seem to be routinely observed as turning green by the public and even severe storm researchers when out conducting experiments, there may be a number of different reasons for the green color of storm clouds. It may be 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 it may be that storm clouds act as a kind of canvas upon which low sunlight scattered by particles in the air paints a greenish color. These explanations 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. 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. Green and other color clouds were found in thunderstorms dropping hail. This seems to discount the notion that ice within the clouds creates the green color.

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. Our advice: don't wait to see green clouds before seeking shelter from severe thunderstorms.