Actually, they can measure some pretty nifty things about electrical current from down at the powerhouse.
Go to your local one and ask when/if they schedule tours.
I and my family got in to our local town's power plant and the got a huge earful.
Basically, from what the electrician said, if you're drawing enough power to do 0.25 horsepower of work, they can detect it.
Good fellow; he was a lineman on the huge lines that go between towns before he settled down in my town with his wife.
Also, as far as humidity causing loss of power in the lines..
There's a HUGE difference between electricity being impeded by an induction coil (causing resistance on the line in order to move the charge) and the air being wet and mucking with the line.
Wet air doesn't cause electrical resistance - wet air in theory can cause a short.
On the big lines like you're talking, they've been designed to handle big rains and wee tornadoes - a bit heavier duty for line shorting than your 'air humidity'
Induction coil (resister-like device used to transfer power and scale it) is a 'resistance' thing.
Wet air is an 'electrical short' thing.