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  1. #1
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    Apr 2013
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    How To Calculate RH% ?

    Hi, Please Help To Explain The Following Question

    How To Calculate RH From Celsius Which Show In Mercury?

    I Used 2 Mercury:
    ~First Mercury Covered With Damp Cloth Temperature Show is 21'C
    ~While, Another Mercury Show Room Temperature Which is 26'C.

    How To Calculate The Relative Humidity% (RH) from temperature shows above?

    What is The Formula To Calculate? Please Advice. Thanks...

  2. #2
    Join Date
    May 2012
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    Re: How To Calculate RH% ?


    You need a sliding scale to calculate the RH.

    You line one side on the wet bulb reading
    you line the other side on the dry bulb reading
    and then it tells you the RH of the air measured.

    At 26 dry and 21 wet you would be about 65% RH

    Or you use a Psychrometric chart.



    Last edited by Rob White; 16-04-2013 at 10:51 AM.
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  3. #3
    Brian_UK's Avatar
    Brian_UK is online now Moderator I am starting to push the Mods: of RE Site Moderator : and general nice guy
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    Mar 2001
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    Re: How To Calculate RH% ?

    To find RH the hard way....

    Relative humidity from temperature and dewpoint

    If you know the temperature and the dewpoint, and want to obtain relative humidity, the formulas are as follows:
    First, to convert the temperature and the dewpoint from Fahrenheit to Celsius, use the following formulas.
    (3) Tc=5.0/9.0*(Tf-32.0)
    (4) Tdc=5.0/9.0*(Tdf-32.0)
    Tc=air temperature in degrees Celsius, Tf=air temperature in degrees Fahrenheit
    Tdc=dewpoint temperature in degrees Celsius
    Tdf=dewpoint temperature in degrees Fahrenheit
    Note: If your temperature and dewpoint are already in degrees Celsius, you can skip the first step and proceed to the second.
    The next set of formulas assumes a standard atmospheric pressure. These formulas will calculate saturation vapor pressure(Es) and actual vapor pressure(E) in millibars.
    (5) Es=6.11*10.0**(7.5*Tc/(237.7+Tc))
    (6) E=6.11*10.0**(7.5*Tdc/(237.7+Tdc))
    Once you have the saturation vapor pressure and the actual vapor pressure, relative humidity can be computed by dividing the actual vapor pressure by the saturation vapor pressure and then multiplying by 100 to convert the quantity to a percent.
    (7) Relative Humidity(RH) in percent =(E/Es)*100
    For example, if you have a station report that included an air temperature of 85 degrees Fahrenheit and a dewpoint of 65 degrees Fahrenheit and you wanted to compute the relative humidity, you would proceed as follows.
    First, convert the Fahrenheit values to Celsius using formulas (3) and (4). The values you get should be Tc=29.4 and Tdc=18.3
    Next, calculate the saturation vapor pressure and the actual vapor pressure using formulas (5) and (6) respectively. The values you get should be Es=40.9 and E=21.0

    Finally, calculate relative humidity using formula (7). The final answer should be RH=51.3 %(percent).
    Note: Due to the rounding of decimal places, your answer may be slightly different from the above answer, but it should be within 2%.

    Brian - Newton Abbot, Devon, UK
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