There has been some discussion in the comments about Hermann Harde's papers purporting to show that carbon dioxide is the tiniest knob controlling the climate. Well, bunnies know that Richard Alley differs, but Eli's point was that Harde overestimates the relative humidity at altitude and this obviously increases the role that water vapor plays in his calculations (make no mistake it plays a huge role in real life) and underestimates the effect that carbon dioxide plays in the greenhouse effect.
Just to be clear, Eli does agree with Pekka and Tom that the text dealing with how to calculate raditative transfer in the atmosphere is a fine introduction. However, long years of gazing at complex spectra lead Eli to see clearly that the water vapor peaks in Harde's calculated emission spectra were way too high.
Some doubts have been expressed.
To put these at rest, first look at Figure 1 from Harde's paper in teh Open Journal of Atmospheric and Climate Change
The interesting thing is why, and the clue to that can be found in the article which Ray Pierrehumbert wrote and which Eli pointed to in the comments to the original Wet Post, but not the part that he quotes here
So what really determines the water vapor content of the free troposphere? It is easiest to think about this problem in a Lagrangian sense, tracking the water content of an air parcel as it wanders about the atmosphere. The fluctuating water content of the parcel results from a balance between the rate at which water is added to the parcel against the rate at which water is removed. Water vapor is removed either by condensation or by diffusion into a neighboring drier air parcel. Let us suppose for the moment that diffusivity is so low that the latter mechanism is unimportant. In that case, water vapor is removed when the air parcel wanders into a region where the local saturation specific humidity is lower than the current specific humidity of the parcel, at which time the specific humidity is reset to the lower local saturation value and the balance is rained out. The net result is that the specific humidity of an initially saturated parcel after time τ is equal to the minimum encountered along the trajectory during that time. By definition, this is a non-increasing function of τ, though there will be long periods of time over which the minimum remains constant between those times at which new minima are encountered.Any attempt to use equilibrium thermodynamics to calculate relative humidity in the troposphere outside the marine boundary layer (where it is saturated) is doomed.