green sky???

[wx1028]

why sometimes during a severe thunderstorm the skty tends to have a greenish tint to it??

[brunota2003]

from what I've read, it has to do with the way the light of the sun bounces off of hail and precipitation...

[coriolis]

I heard that's a sign of a severe storm.

[DrCloud]

Neither of these is really an explanation, and I haven't found such a thing that I can find a link to.

Molecular (Rayleigh) scattering creates the blue sky, because the air molecules are the right size to scatter preferentially the shorter wavelengths of the solar spectrum. Longer pathlengths (when the sun is rising or setting) tend to absorb the short wavelengths, and very fine particles (haze) preferentially scatter red wavelengths, which explains spectacular sunsets.

Green, though, is more in the middle of the spectrum and preferential scattering is an unlikely explanation. One source discussed in quite vague terms the possibility of ice-crystal refraction causing the green appearance of impending thunderstorms, but that sounds an awful lot like handwaving.

Well, if they can handwave, so can I:

Thunderstorms are sufficiently deep and wet (in the sense of a very long water-drop pathlength) that little sunlight penetrates them to get to the ground. The base of the storm, which can be quite flat if strong downdrafts haven't reached that far down yet, is therefore illuminated by reflected light. Depending on the time of day and the geometry, this can easily be direct sunlight reflected from the ground.

In particular, the green appearance seems often to be associated with the leading edges of storm systems. Maybe there's an anvil way up there, but often there is sunlight on the ground nearby. In tornado alley (and the green appearance is something that tornado watchers talk about), the ground is green, generally, so the reflected light that's illuminating the cloud base is green light. Just as cloud tops or the sidewalls of deep storm clouds appear white, because the drops and crysals are big enough to scatter all colors about equally, whatever light that's illuminating the cloud base will also be reflected according to its mix of incoming colors. If that's green light, well, the reflection will be green light. Hence a green cloud.

Of course, this is imprecise (the ground is not a uniform green; there is sidewall penetration of direct sunlight, etc.) so the cloud base is never bright green like trees or grass. But it looks greenish.

A test of this would be to ask if there are ever green-looking clouds over surfaces with a predominance of other color. HPH

[wxman57]

You might check out this paper. The author performed measurements of green thunderstorms and determined the color is not a foilage reflection:

http://cat.inist.fr/?aModele=afficheN&cpsidt=1060493

And a good article here explaining the phenomenon:
http://www.usatoday.com/weather/resourc ... storms.htm

I've observed many green thunderstorms, and I've always taken it to be a strong indicator of large hail.

[DrCloud]

Good find.

And so much for handwaving. The Gallagher paper in J. Appl. Met. shoots that down pretty thoroughly -- and I might have known that the basic explanation would have originated with Craig Bohren.

But what the analyses show is that it's related to the water content of deep clouds rather than hail per se -- of course the two go together often, so green clouds can provide a reasonable symptom of impending severe weather. HPH

[JonathanBelles]

from my experiance green sky is a sign of extremely hard rain or an intes thunderstorm. red sky means a tornado is in the area. yellow sky comes after a heavy rain in the afternoon. white sky means the fog is rolling or has rolled in. blue sky is good.

[artist]

in north central Tx a green sky was a visual warning to us old fogies there that it was going to be an extreme thunderstorm with large hail or possibly a tornado.