I graphed specific humidity since 1940 against sunspot number averaged over the solar cycle, and got this surprising result:
Since sunspot numbers don’t correlate so well as this with temperature, it raises the question of what it is which controls specific humidity in the atmosphere. How might the solar flux be affecting humidity?
Wikipedia says this about specific humidity:
Specific humidity is the ratio of water vapor to air (including water vapor and dry air) in a particular mass. Specific humidity ratio is expressed as a ratio of kilograms of water vapor, mw, per kilogram of total moist air mt .
That ratio can be shown as:
SH = mw / mt
Specific humidity is related to mixing ratio (and vice versa) by:
SH = omega / 1+omega
omega = SH / 1 – SH
Update: The correlation looks even stronger with the sunspot numbers smoothed at the length of the Earth-Venus cycle, 96 months. It also looks strong at around 82 months, half the length of the Jupiter-Uranus synodic period which coincides with flooding events, as noted by Ulric Lyons in comments.
Divergences coincide with el nino events and the rebound la nina aftermath.
Update 2: The Specific Humidity data on the above graphs is for the atmosphere at around 30,000 feet. Just a bit higher than Mt Everest, and around the height airliners fly at. It is also around the height of the tropopause: the troposhere – stratosphere interface, at the poles. The curves look different at other altitudes, so our solar correlation seems to be specific to this level in the atmosphere. This might give us a clue as to the ways energy is propogated in the climate system.