Lasers do not obey the inverse square law. As Adam described short wavelength lasers (blue or UV) are most suseptible to atmospheric power loss. SDI type lasers use lines in the 2000 nm and the 4000 nm range, way past infared, and completely invisible.
For less than $1000 in parts you can build a NdYAG delivering ten's of watts, the 1064 nm beam is not visible at all. A KTP crystal can inexpensively frequency double that to a lime greem 532 nm.
A good quality collimating lens or beam expander would keep the beam focused over many miles. You won't ablate metal, but you could cause serious eye damage.
Laser injures Delta pilot's eye:
http://washingtontimes.com/national/20040928-111356-3924r.htm
Right. The green would be very dazzling as well as destructive. However the 1064 is also a good propagator as well as very dangerous to the eyes.
A good quality collimating lens or beam expander would keep the beam focused over many miles. You won't ablate metal, but you could cause serious eye damage.
Yep. Mode quality takes some work but is not too hard.