Well I don't know where the idea or believe (?) that you can see nuclear radioactivity as greenish or yellowish actually came from but as far as I know you can't see radioactivity as a color of any kind ??Yeah me too Pashalis, but let's not forget that Russia experienced huge fires: http://www.sott.net/articles/show/231271-Political-Bread-and-Circuses-for-the-Masses-While-the-World-is-Burning-Russia-s-Summer-of-Fire-Intrigue-Political-Mystery
This fire happened last year in July/August (2011), so I'm not sure if it still could cause any green clouds.
So far I couldn't find anything of green clouds some years before, so it happens eventually not that often and could be a rare phenomenon, if at all. But this is just the term green clouds.
I could find one blog:
Beware the green clouds
By KEN | Published: APRIL 2, 2006
So a few moments ago the Community Wireless Summit was wrapping up and a bunch of us were hanging out saying our goodbyes. In the course of 5 minutes a beatiful sunny day with people going shirtless turned into a complete grey sky with large green clouds rolling through fast enoughÂ to makeÂ everyone turn around and look. In the courseÂ of 30 seconds a strong gust turned it to screaming winds. Tornado I thought?? Really loud sirens started sounding, we all scattered to the nearset building waiting for someone with a key to the dorm building to let us in.
So now we are sitting in basement of a dorm at Lindewood University listening to the sound doors clacking and recounting what for a lot of us is our first potential tornado. The quickness with which the storm took over a completely sunny normal day was amazing. It was really just long enough to for you to start thinking “Is this a tornado wind?”, by the time the thought crossed your mind your running.
I don't know if it is meant literally in this case or not.
And from wiki:
Green Clouds - Green clouds are normally seen when there is a large amount of ice in the clouds. This is an indication of hail and could be a warning that a tornado is likely.
Also some newssites report that it isn't an alien invasion.