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How to calculate amount of dew per unit area.

Innocent Muggle

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2 hours ago, Innocent Muggle said:

Hello al! :)

I wonder how to calculate amount of dew in the morning per unit area.

set the temperature go down 20c from 40c and unit area is 1m square.


I hope some one explain the formula for this!

thank you!

How long have you got?

Firstly, you need to know the water carrying capacity of air at your lowest temperature.

This is calculated via the saturated vapour pressure (Ps1) of water at this temperature (T1) for which the August Roche Magnus equation is probably most convenient and simplest (it derives from the Clausius Clapeyron equation qv)

Ps1 = 610.94e^(17.635(T-273.15)/(T - 30.11))

Where units are Pa and K. For T = 293.15 (20 oC) this gives Ps1 = 2,335 Pa

The ideal gas equation (PV = nRT) then tells us that the maximum molar water vapour capacity per m3.

ns1 / V = Ps1 / RT = 2,335 / (8.3145 x 293.15) = 0.9573 gmol / m3 

Taking 18 as the mol wt of water that gives a mass density of 18 x 0.9573 = 17.23 g / m3 @ 20 oC.

That's more than enough for one post.

Tell me if you follow it so far and we can move on to an easy bit followed by a hard bit. 


Edited by sethoflagos
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