green sky

[vanilla]

Why, when theres going to be a bad storm, does the sky take on a weird greenish tint?

[Bosda_Di_Chi_of_Tricor]

I have no idea why, but I’ve seen it happen when weather is at it’s worst.

IMHO–dust in the air?

[yabob]

A “USA Today” article which seems surprisingly factual, and provides some links to scientific papers at the bottom. Unfortunately, only the last of the three seems to work:

http://www.usatoday.com/weather/severe/wgreensky.htm

The third and fourth paragraphs sum up two major theories:

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.

Another possibility is 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.

The abstract of the scientific paper linked to at the bottom mentions both of these as well, and says they may not be mutually exclusive - both effects may be at work.