From Glossary of Meteorology
Any thunderstorm that is perceived by observers to be green.
The perceptually dominant wavelength of light from green thunderstorms ranges from blue- green to yellow-green. The purity of the color is generally low and the physical mechanism that causes the green appearance is not understood. Although green clouds often occur in conjunction with severe weather, there is no evidence to support anecdotal attributions of the cause of this green to specific characteristics of severe storms, such as hail or tornadoes.
Bohren, C. F., and A. B. Fraser 1993. Green thunderstorms. Bull. Amer. Meteor. Soc.. 74. 2185–2193.