Subject: Dewpoint

# Dewpoint

From: Walter Henry <whenry>
Date: Monday, August 26, 1991
```The following discussion took place in sci.geo.meteorology and is
reposted here without the knowledge or permission of the authors.

Article: 1019 of sci.geo.meteorology
From: moonunit [at] meteor__wisc__edu (Chris Bovitz)
Newsgroups: sci.geo.meteorology
Subject: Dewpoint
Date: 21 Aug 91

In article <1991Aug21.095313.32841 [at] kuhub__cc__ukans__edu>
arritt [at] kuhub__cc__ukans__edu writes:
>In article <1106 [at] uswnvg__UUCP>, cjackso [at] uswnvg__UUCP (
>               Clay Jackson) writes:
>> Does anyone have the formula for obtaining dewpoint from RH and
>> dry bulb temp (and vice versa)?
>
>The following article is very useful for this type of problem. It
>also has formulations for the lifting condensation level, equivalent
>potential temperature, etc., all to very high accuracy.

Bolton, D., 1980: The computation of equivalent potential temperature.
Monthly Weather Review, 108, 1046-1053.

In the _Smithsonian_Meteorological_Tables_, there is a formula which
will give you the actual vapor pressure (use dew point temp) and the
saturation vapor pressure (use dry bulb temp).  Since RH = E / Es (where
E = vapor pressure, Es = saturation vapor pressure), you know any two,
you can get the third.

If you use the Sixth Revised Edition (1951), there is an error in the
Goff-Gratch equation's second-to-last term: it should be multiplied by
10^-3.

Chris

--
Chris Bovitz              | Twins Update: (08/20)  beat SEA 10-5
Department of Meteorology |     AL West:   MIN  72-49   --
Univ of WI - Madison      |                CHI  67-53   4.5
moonunit [at] meteor__wisc__edu  |                OAK  66-55   6

Article: 1021 of sci.geo.meteorology
From: 2me [at] mace__cc__purdue__edu (M Bosilovich)
Newsgroups: sci.geo.meteorology
Subject: Dewpoint
Date: 21 Aug 91

...
I'm rooting through my memory, but I think that the Clausius-Clapeyron
equation is what you require. It has many forms but this should help

ln(es/e) = L/Rv * (1/Td - 1/T)

e/es represents the RH, Rv = 461 J/kgK,  L=2.5x10^6 J/kg.

This is a very useful form.

You may want to double check this with some meteorological textbook, as
I did this note off the top of my head.

Mike B.    2me [at] mace__cc__purdue__edu
Purdue University  Dept. of Earth and Atmospheric Science

Article 1050 of sci.geo.meteorology:
From: JG68 [at] LIVERPOOL__AC__UK
Newsgroups: sci.geo.meteorology
Subject: Dewpoint
Date: 23 Aug 91

On dew point from RH and T dry

see, Sargent, G.P. (1980). Computation of vapour pressure, dew-point
and relative humidity from dry_ and wet_bulb temperatures,
METEOROLOGICAL MAGAZINE, 109, 238-246.

within certain ranges the following may be used

Td = U(0.198 + 0.0017T) + 0.84T -19.2

where Td is dew point temperature Celsius, T is dry bulb Celsius and U
is relative humidity, per cent (%).

this equation is supposed to give T dew values within 1 C of the true
value over the temperature range 0 C to + 30 C and RH values of 100
per cent to 40 per cent

Hope this is of some help.

Dr. Andy Morse,                JG68 [at] UK__AC__LIV
Department of Geography,
University of Liverpool,
U.K.

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Received on Monday, 26 August, 1991