Estimating Average Rainfall In a question related to my previous one. I am in the process of building a fantasy world. I have the landmasses with elevation worked out along with the air/wind directions at the various latitudes and the temperature both average and seasonal. 

What I am missing in determining the climates is the rainfall. I have a rough idea of probable rainfall as humid air goes inland based on elevation changes and temperature. However, if someone could provide a more exact formula or answer to exactly how much average (daily or monthly or whatever) rainfall should occur assuming the air mass hits the land carrying a certain amount of grams of water per cubic meter. For example at 20 deg centigrade air can hole 17.3 grams per cubic meter at 100% humidity. I may be incorrectly assuming rainfall will be largely based on how much altitude the air has to climb as it moves inland.

Second to that is what rate does the air mass gain humidity as it travels over water. 

Imagine I have a air mass moving over ocean then over land and then perhaps back over another stretch of water before hitting land again. Those are the type of issues I am facing in trying to determine the climate
Probable rainfall is constrained by several factors, both spatial and temporal. Rainfall modelling is in itself a humongous topic. You may refer to this link for having a good idea of the simulation process &amp; maybe adapt some of the basic equations for a rough estimate by assuming some parameters as minimally affecting your site.