formula for relative humidity from dewpoint and tempetature
formula for relative humidity from dewpoint and tempetature
(OP)
Is there and approximate formula that can compute relative humidity from dewpoint and tempetature?
Thanks
Pete
Thanks
Pete





RE: formula for relative humidity from dewpoint and tempetature
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.
RE: formula for relative humidity from dewpoint and tempetature
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
RE: formula for relative humidity from dewpoint and tempetature
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
RE: formula for relative humidity from dewpoint and tempetature
RE: formula for relative humidity from dewpoint and tempetature
# 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.
RE: formula for relative humidity from dewpoint and tempetature