<p><a href="https://www.engineeringtoolbox.com/evaporation-water-surface-d_690.html" rel="nofollow noreferrer">https://www.engineeringtoolbox.com/evaporation-water-surface-d_690.html</a></p>

<p>The amount of evaporated water can be expressed as:</p>

<p>gs = Θ A (xs - x) / 3600                                 (1)</p>

<p>or</p>

<p>gh = Θ A (xs - x)</p>

<p>where</p>

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

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

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

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

<p>A = water surface area (m2)</p>

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

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

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

<p>Found the answer to my question.</p>

CC BY-SA 4.0
2018-06-04T17:57:28.373Z
2018-06-04T17:57:28.373Z
C Viau
<p>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?</p>

<p>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.</p>

<p>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.</p>

CC BY-SA 4.0
2018-06-02T10:49:25.860Z
The complication is that it will fluctuate through the day.  During the afternoon, the air warms, rises and mixes in the [PBL](https://en.wikipedia.org/wiki/Planetary_boundary_layer).  So it's just the surface layer you must "fill", another complication.
JeopardyTempest
CC BY-SA 4.0
2018-06-02T11:04:57.650Z
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](http://www.pivotalweather.com/model.php?m=gfs&p=sfcrh&rh=2018060206&fh=loop&r=eu_sc&dpdt=) (depending on airmass intrusions), and the NA Great Lakes [do a fair job of keeping air moist.](http://www.pivotalweather.com/model.php?m=gfs&p=sfcrh&rh=2018060206&fh=loop&r=conus&dpdt=).
JeopardyTempest
CC BY-SA 4.0
2018-06-02T11:38:54.307Z
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](https://www.climate-charts.com/World-Climate-Maps.html#rain) 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).
JeopardyTempest
CC BY-SA 4.0
2018-06-02T11:47:56.963Z
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](https://earthscience.stackexchange.com/questions/42/what-are-monsoons-caused-by), 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!)
JeopardyTempest
CC BY-SA 4.0
2018-06-02T12:35:27.993Z
As a whole, a great deal of precipitation regimes has to do with pressures. Pressure differences control the winds, and upper scale patterns also modulate the propensity for lift. A great deal of that would be driven by [the location/number of global circulation cells](https://en.wikipedia.org/wiki/Atmospheric_circulation) your world would have. And how that lines up with ocean currents and terrain. Soil type/vegetative cover will mean a lot, which may have to do with changes over eons.
JeopardyTempest
CC BY-SA 4.0
2018-06-02T12:36:01.670Z
In the end, the answer unfortunately for the bigger picture of what would weather look like on another world is unfortunately that it's too complex to give useful simple answers. You can certainly try looking at the things mentioned to try to give a rough more realistic idea of rainfall. But so much is due to complex interactions between various features, that to get a true realistic picture would take changing the world in a climate model, [and that's not all that easy either](https://physics.stackexchange.com/questions/281195/where-to-start-if-i-would-like-to-run-a-planet-climate-simulation)
JeopardyTempest
<meteorology>
Ocean air reaching maximum humidity
C Viau
# Ocean air reaching maximum humidity

(Asked by: username_0 on 2018-06-01)

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.

**Tags:** meteorology

> **Comments:**
> **username_1**: The complication is that it will fluctuate through the day.  During the afternoon, the air warms, rises and mixes in the [PBL](https://en.wikipedia.org/wiki/Planetary_boundary_layer).  So it's just the surface layer you must "fill", another complication.  
> **username_1**: 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](http://www.pivotalweather.com/model.php?m=gfs&p=sfcrh&rh=2018060206&fh=loop&r=eu_sc&dpdt=) (depending on airmass intrusions), and the NA Great Lakes [do a fair job of keeping air moist.](http://www.pivotalweather.com/model.php?m=gfs&p=sfcrh&rh=2018060206&fh=loop&r=conus&dpdt=).  
> **username_1**: 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](https://www.climate-charts.com/World-Climate-Maps.html#rain) 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).  
> **username_1**: 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](https://earthscience.stackexchange.com/questions/42/what-are-monsoons-caused-by), 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!)  
> **username_1**: As a whole, a great deal of precipitation regimes has to do with pressures. Pressure differences control the winds, and upper scale patterns also modulate the propensity for lift. A great deal of that would be driven by [the location/number of global circulation cells](https://en.wikipedia.org/wiki/Atmospheric_circulation) your world would have. And how that lines up with ocean currents and terrain. Soil type/vegetative cover will mean a lot, which may have to do with changes over eons.  
> **username_1**: In the end, the answer unfortunately for the bigger picture of what would weather look like on another world is unfortunately that it's too complex to give useful simple answers. You can certainly try looking at the things mentioned to try to give a rough more realistic idea of rainfall. But so much is due to complex interactions between various features, that to get a true realistic picture would take changing the world in a climate model, [and that's not all that easy either](https://physics.stackexchange.com/questions/281195/where-to-start-if-i-would-like-to-run-a-planet-climate-simulation)  

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## 1 answers

### Answer by username_0 on 2018-06-04. Score: 2

<https://www.engineeringtoolbox.com/evaporation-water-surface-d_690.html>

The amount of evaporated water can be expressed as:

gs = Θ A (xs - x) / 3600 (1)

or

gh = Θ A (xs - x)

where

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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