Slowboat - You are correct in understanding that light requires a reflective surface to be seen. In the case of lasers, the reflectivity results from particulates in the air through which the beam passes.
'Red' laser beams appear 'invisible' because the their wavelength is up in the 740nM range, near the end of the spectrum visible to the human eye. In other words, our eye doesn't see the visible beam as easily, therefore it appears less visible. By contrast, 'green' lasers emit a beam in the 550nM range, which is right in the middle of the frequencies which the human eye perceives, making it most visible.
Thus, clouds only provide a reflective 'end point' for a laser. Our atmosphere, itself, acts to make the beam visible, which makes a green laser ideal for uses as varied as pointing out stars, or as a highly controllable 'flare' to attract the attention of your ride home at O'dark-thirty... Besides, they're fun to play with. I bet you didn't know that you can make a glass of beer in a bar glow from across the room!
sub6
Thanks, subsea. Learning more about this stuff every day. If I didn't think it would get me arrested, I'd go find a green laser and play with it tonight. What the heck, maybe I will anyway.