There is no back radiation...water vapor keeps it warm by actually absorbing and retaining energy..a trick that CO2 can't do...that is why it is warmer at night in humid begins than it is in dry regions...nothing whatsoever to do with imaginary back radiation.
Can you give us a link or source of that?
On top of everything else, now you tell me that you are unaware of the fact that water vapor actually absorbs and retains energy...tell me that so I can put one more mark in the wuwei doesn't have a clue column...
Go learn about it yourself...