How many miles (or how many hours/days) does air have to travel over the ocean to reach maximum humidity or carry the maximum amount of water it would hold at typical humidity assuming a wind speed of roughly 10 miles per hour?

I understand the typical humidity to be roughly 80% over the oceans. I have a fantasy world with its own landmasses and I'm trying to be able to figure out the climate based on temperature, wind direction and rainfall at different latitudes.

Take North Africa versus the same Latitude in China. At 30 degrees latitude you don't always get a desert climate. If the wind can travel over water it will bring in some rainfall but I'm trying to understand how much depending on the answer to the above question.

  • 1
    $\begingroup$ depends on SST, time of year, ocean state.but good luck in getting an answer for this one. $\endgroup$
    – user1066
    Jun 2, 2018 at 4:15
  • $\begingroup$ The complication is that it will fluctuate through the day. During the afternoon, the air warms, rises and mixes in the PBL. So it's just the surface layer you must "fill", another complication. $\endgroup$ Jun 2, 2018 at 10:49
  • $\begingroup$ You are right, though, it depends upon the resident time over the body of water. Rather than complex calculations, perhaps looking at various bodies of water can help... the Mediterranean Sea generally keep air pretty saturated (depending on airmass intrusions), and the NA Great Lakes do a fair job of keeping air moist.. $\endgroup$ Jun 2, 2018 at 11:04
  • $\begingroup$ The thing is, even then rainfall can be pretty seasonal. Wind fluctuates a great deal in most places not in the deep tropics. This site looks to offer some fair global climate maps (though you have to zoom some of the images a lot and they're pretty low-res). $\endgroup$ Jun 2, 2018 at 11:38
  • $\begingroup$ The thing is, places in like India and SE China, despite wind directions that may be predominantly offshore, can still manage tons of rain due to the onshore winds lining up during the right season and pressure backgrounds (which can be controlled by much larger complexities). And as gansub suggested on this SE question about monsoons, perhaps they're pretty complex to answer. I'd hint that a lot has to do with ocean circulations (so not just moisture, but pressure favors east coasts for rain, but not always!) $\endgroup$ Jun 2, 2018 at 11:47

1 Answer 1



The amount of evaporated water can be expressed as:

gs = Θ A (xs - x) / 3600 (1)


gh = Θ A (xs - x)


gs = amount of evaporated water per second (kg/s)

gh = amount of evaporated water per hour (kg/h)

Θ = (25 + 19 v) = evaporation coefficient (kg/m2h)

v = velocity of air above the water surface (m/s)

A = water surface area (m2)

xs = maximum humidity ratio of saturated air at the same temperature as the water surface (kg/kg) (kg H2O in kg Dry Air)

x = humidity ratio air (kg/kg) (kg H2O in kg Dry Air)

Note! The units for Θ don't match since the this is an empirical equation - a result of experience and experiments.

Found the answer to my question.


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