Subject: How strong would a laser have to be to be clearly visible in the air?

Now, knowing that our atmosphere has a certain dielectric strength that a laser 
would have to overcome, what would be required, minimally, for a laser to be 
clearly seen through the air?  How about underwater?  I know that 
the 'atmosphere' of space is only a few hydrogen atoms/square centimeter, but 
would it be physically possible for a laser to become strong enough to be seen 
there, too?

Re: How strong would a laser have to be to be clearly visible in the air?