I guess any type of precipitation.. for one
dust, smoke, ash?
(lets just say any type of particulate matter floating about) 

Good answer! Almost anything suspended in the air would create static electricity on the aircraft. I have seen it so bad, you could wipe it off your arms, be all over instrument panel and etc. Interesting thing about it was, if you got close enough to a thunderstorm cel, sometimes you would have a "static discharge", which a lot of people thought was a lightening strike, but was in essence just a ground for the static to jump off on as the lightening bolt struck the ground. Had one, one night on the way to Porta Rica, in a DC-6B! Tail cone, where it all exited the aircraft, had what looked like a 1,000 little holes in it and it had to be replaced. Lightening does not struck aircraft, unless it happens to be in the way as the lightening bolt goes to ground. Its almost always a static discharge from the airframe!