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Green Thunderstorms

Craig F. BohrenDepartment of Meteorology, The Pennsylvania State University, University Park, Pennsylvania

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Alistair B. FraserDepartment of Meteorology, The Pennsylvania State University, University Park, Pennsylvania

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Green thunderstorms are observed occasionally, yet with one exception they have received no scientific attention, experimental or theoretical. Fraser suggested that thunderstorms themselves are not green but that a thick thunderstorm provides a dark backdrop for green airlight near sundown. Greenness is a consequence of reddened sunlight illuminating selective scatterers along the observer's line of sight. Bohren's alternative explanation is that green thunderstorms may be a consequence of the intrinsic blueness of clouds because of selective absorption by pure water, liquid or solid. Most clouds are so thin that the light transmitted by them is not markedly colored because of selective absorption. Only the most massive clouds—large both vertically and horizontally—are thick enough to shift the color of incident sunlight upon transmission. If that incident light is sunlight reddened at sundown, the transmitted light can be perceptually green. These two explanations do not exclude one another but allow for multiple causes, including those not yet identified.

Green thunderstorms are observed occasionally, yet with one exception they have received no scientific attention, experimental or theoretical. Fraser suggested that thunderstorms themselves are not green but that a thick thunderstorm provides a dark backdrop for green airlight near sundown. Greenness is a consequence of reddened sunlight illuminating selective scatterers along the observer's line of sight. Bohren's alternative explanation is that green thunderstorms may be a consequence of the intrinsic blueness of clouds because of selective absorption by pure water, liquid or solid. Most clouds are so thin that the light transmitted by them is not markedly colored because of selective absorption. Only the most massive clouds—large both vertically and horizontally—are thick enough to shift the color of incident sunlight upon transmission. If that incident light is sunlight reddened at sundown, the transmitted light can be perceptually green. These two explanations do not exclude one another but allow for multiple causes, including those not yet identified.

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