# 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

• Keep in mind that such a 17.3 g/m$^3$ would actually be Earth-specific, and so you may still have some more freedom - mixing ratios are likely quite dependent upon atmospheric pressure (and so things like gravitational value and atmospheric composition). – JeopardyTempest Jun 2 '18 at 12:13
• Rainfall in some places is connected to elevation changes, but it's not nearly a simple factor. Take a look at the US distribution and elevations. Places near sea-level, like FL, and places at high elevation, like the Colorado Rockies, both have quite a bit. Death Valley is near/below sea-level and gets minimal precip, the Great Basin is rather high, but very dry. – JeopardyTempest Jun 2 '18 at 12:31
• Moist air flowing up sharp terrain features will be rainy places. Places blocked from access to moisture by being deep on the shadow side of mountains from such water bodies will be dry. The (absolute) amount of moisture in air does decrease with elevation, which does explain the east-west gradient of precip across the central US. – JeopardyTempest Jun 2 '18 at 12:33
• (See also my comments on your other question which go into further discussion of the precipitation question as a whole. I guess all you can do is try to weigh these factors as best you can, and try a guess on the layout. I've seen quite a few others ask similar questions through the years, many do want their fantasy world to be as realistic as possible... but short of running geologic and climate models, it's going to be pretty far off. Just make the best of it? :-D – JeopardyTempest Jun 2 '18 at 12:42
• You may find it easier to do a sort of reverse model. Develop a simple relationship between total moisture content, elevation and precipitation rate. Then assign moisture content to the air that gives you a pleasing distribution of precipitation. – haresfur Aug 29 '18 at 22:48