# Re: Vapor Pressure and Saturated Vapor Pressure

From: Dennis Shea <shea_at_nyahnyahspammersnyahnyah>
Date: Wed Apr 13 2011 - 20:25:17 MDT

The following is fortran but writing and NCL functio to
do this would be trivial.

real function esat(t)
c NCLEND

c include 'lib_dev:[gudoc]edfvaxbox.for/list'
c g.s. stipanuk 1973 original version.
c reference stipanuk paper entitled:
c "algorithms for generating a skew-t, log p
c diagram and computing selected meteorological
c quantities."
c atmospheric sciences laboratory
c u.s. army electronics command
c white sands missile range, new mexico 88002
c 33 pages
c baker, schlatter 17-may-1982

c this function returns the saturation vapor pressure over
c water (mb) given the temperature (celsius).
c the algorithm is due to nordquist, w.s.,1973: "numerical approxima-
c tions of selected meteorlolgical parameters for cloud physics prob-
c lems," ecom-5475, atmospheric sciences laboratory, u.s. army
c electronics command, white sands missile range, new mexico 88002.

tk = t+273.15
p1 = 11.344-0.0303998*tk
p2 = 3.49149-1302.8844/tk
c1 = 23.832241-5.02808*alog10(tk)
esat = 10.**(c1-1.3816e-7*10.**p1+8.1328e-3*10.**p2-2949.076/tk)
return
end

or

C NCLFORTSTART
real function es(t)
c NCLEND

c this function returns the saturation vapor pressure es (mb) over
c liquid water given the temperature t (celsius). the formula appears
c in bolton, david, 1980: "the computation of equivalent potential
c temperature," monthly weather review, vol. 108, no. 7 (july),
c p. 1047, eq.(10). the quoted accuracy is 0.3% or better for
c -35 < t < 35c.

c include 'lib_dev:[gudoc]edfvaxbox.for/list'
c baker, schlatter 17-may-1982 original version.

c es0 = saturation vapor pressure over liquid water at 0c

data es0/6.1121/
es = es0*exp(17.67*t/(t+243.5))
return
end

```---
If you have the dw point temperature you can solve for the vapor
pressure using the following
C NCLFORTSTART
real function dewpt(ew)
c NCLEND
c   this function yields the dew point dewpt (celsius), given the
c   water vapor pressure ew (millibars).
c   the empirical formula appears in bolton, david, 1980:
c   "the computation of equivalent potential temperature,"
c   monthly weather review, vol. 108, no. 7 (july), p. 1047, eq.(11).
c   the quoted accuracy is 0.03c or less for -35 < dewpt < 35c.
c       include 'lib_dev:[gudoc]edfvaxbox.for/list'
c       baker, schlatter  17-may-1982     original version.
enl = alog(ew)
dewpt = (243.5*enl-440.8)/(19.48-enl)
return
end
On 4/13/11 6:08 PM, jrbuzan wrote:
> I am attempting to calculate Vapor Pressure and Saturated Vapor
> Pressure, but I have yet to find a function.
> My approach was to use the code from the Relhum function.  However,
> when I search through the 'contributed.ncl' file, the only calculation
> I can find for Relhum was with dew point temperature.
>
> Yet, the Relhum function calls Pressure, Temperature, and Mixing Ratio.
>
> Any help?
>
> -Jonathan
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```
Received on Thu Apr 14 12:25:26 2011

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