I am busy with the development of an "environment" model that can be used for flight simulation of all kinds of flight vehicles — ranging from quad-copters, unmanned aerial vehicles, manned aircraft, up to rocket launchers for satellites. The model consists of sub-models for terrestrial gravity; the earth's size, shape, and motions; the geomagnetic field; the atmosphere; and so on.
Currently I am using the COSPAR International Reference Atmosphere (CIRA-86) to calculate air pressure and temperature for heights up to 120 km. But, to be able to calculate air density, I also need to model the effects of humidity. I know that water vapour is only present in varying quantities in the troposhere, and that high humidity is therefore more likely at lower altitudes.
Does anybody have models of the probability of experiencing some value of relative humidity as a function of height above sea level?
Edit. As an example of what I really need, the MIL-F-8785C model for wind speed and turbulence, for example, gives the mean wind speed at a height of 6.1 m above the ground, given the logarithm of the probability of exceeding the wind speed. So, a model of the humidity as a probability of exceeding the the mean humidity as a function of height is what I really need. Do you know of any models that provide this?