I would think that 50% moist air would block your vision after maybe a few miles
http://en.wikipedia.org/wiki/Megametre
Capthca: no-brainer
EDIT:Wait, what happened to my poll, i just edited in more options
http://en.wikipedia.org/wiki/Megametre
Capthca: no-brainer
EDIT:Wait, what happened to my poll, i just edited in more options