formula for relative humidity from dewpoint and tempetature 3

[electricpete]

Is there and approximate formula that can compute relative humidity from dewpoint and tempetature?

Thanks
Pete

 

[TD2K]

Look at this link:

http://www.theweatherprediction.com/habyhints/186/

Take your wet bulb and dry bulb temperatures, calculate the saturation water vapor pressure, the RH is the ratio of the two.

 

[ChasBean1]

Pete -

A nice program (downloadable, free) that does it for you:

http://www.pmilligan.net/pmtherm.htm

After downloading and installing, select "PMTherm.exe." Go to "psychro" on the menu and select "psychrocalc." Screen turns up that allows you to enter any two parameters, press "calc" and any other needed psychrometric paramters appear.

Only trouble is that if you have a data set, PMTherm will only do one value at a time. I believe I have a spreadsheet at work with the equation built in - email me: cbean@eheinc.com

 

[electricpete]

TD2K - I think I knew that once but it has been awhile. So now the problem turns to one of calculating vapor pressure from temperature. Is there any easy formula for that? I'm looking to plug it into a spreadsheet.

cb - It looks like PMTherm won't help because I want to do it in a spreadsheet. I'll e-mail you to see if you have the spreadheets handy.

Sorry if I'm being a litte lazy. I'm sure this info is easily available somewhere on the internet. I haven't searched very hard yet, but I figured you guys would help me get there a little faster.


any other thoughts?
Thanks

 

[TD2K]

ElectricPete, drop me an email at testdog2000@yahoo.com, I've got it already in a spreadsheet if you want to cheat ;-).

 

[electricpete]

Thanks TD2K and CB for sending me spreadsheets. Both gave the same answer for a test case. TD2K's was a little easier for me to understand. Below are the equations that I extracted from his spreadsheet:

# Symbols are as follows:
> # TwbF - wet bulb temperature in degrees F
> # TdbF - dry bulb temperature in degrees F
> # TwbK - wet bulb temperature in degrees K
> # TdbK - dry bulb temperature in degrees K
> # Vwb - Vapor pressure associated with saturation conditions at wet bulb temperature.
> # Vdb - Vapor pressure associated with saturation conditions at dry bulb temperature.
> # RH - Relative Humidity

> TwbK:=(TwbF-32)*5/9+273;
TwbK := 5/9 TwbF + 2297/9
> TdbK:=(TdbF-32)*5/9+273;
TdbK := 5/9 TdbF + 2297/9
> Vwb:=6.11*exp((2453000/461)*(1/273-1/TwbK));
Vwb := 6.11*exp(2453000/125853-2453000/461/(5/9*TwbF+2297/9))
> Vdb:=6.11*exp((2453000/461)*(1/273-1/TdbK));
Vdb := 6.11*exp(2453000/125853-2453000/461/(5/9*TdbF+2297/9))
> RH:=Vwb/Vdb;
RH := exp(2453000/125853-2453000/461/(5/9*TwbF+2297/9))/exp(2453000/125853-2453000/461/(5/9*TdbF+2297/9))
> RH:=simplify(Vwb/Vdb);
RH := exp(239446.854663774*(TwbF-1.*TdbF)/(5.*TwbF+2297.)/(5.*TdbF+2297.))

The above expression for RH is suitable for plugging directly into an excel formula if you change the TwbF and TdbF to cell references.
An example calculation using TwbF=80,TdbF=90, yields RH= .724

Thanks again.

 

[electricpete]

Looking a little closer, I see this is simply an implementation of the equations given at TD2K's link, above.