lee wrote on Feb 9
th, 2016 at 10:41am:
John_Taverner wrote on Feb 8
th, 2016 at 10:18pm:
Do you understand that the atmosphere gets cooler and thinner with height, so that its capacity to hold water vapour decreases substantially as the water dewpoint decreases?
Yes. Although the dwpoint is not a constant.
At an altitude of 35,000 to 40,000 feet, the air pressure is about 1/5 of that at sea level. The temperature is typically about -40 to -50 degrees C and the water vapour content under those conditions is usually 2/5 of FA. (Forecast Analysis of course. What did you think I meant?)
At sea level at 100% Relative Humidity and say 20 degrees C, the water content is about 17 grams per cubic metre of air.
At -50 degrees, even at 1 bar, and 100% RH, the water content is 0.038 grams per cubic metre of air. At 0.2 Bar, it's considerably less than that. You are correct. It varies, but on average, it's much less than 0.038g/m3, or < 0.22% of its value at sea level at 1 bar. This is a grossly conservative exaggeration. In practice, there is an adiapatic lapse associated with dew point against altitude (it decreases)
So you can see that as altitude increases, water vapour becomes less important as a greenhouse gas, because the air is extremely dry. Carbon dioxide, methane and all the other fixed gases still have roughly the same concentration in ppm, but their vapour pressure falls in line with barometric pressure. At 50,000 feet, it's roughly 1/5.
So if you look at the absorption spectrum in the Lower Troposphere, the water peaks tend to be broader due to higher concetration. In the lower stratosphere, Carbon dioxide is the dominant greenhouse gas. (and when it comes to radiation to space, the lower stratosphere/ upper troposphere is where most of the action is. )
Does this still sound like magic, or have you grasped the principle? I'm trying to keep it as non-technical as possible.