Dewpoint Formulas

  Dewpoint calculated from:    Dry Bulb Temperature 
                               Relative Humidity
-----------------------------------------------------------------------------

     B = (ln(RH / 100) + ((17.27 * T) / (237.3 + T))) / 17.27

     D = (237.3 * B) / (1 - B)

       where:
               T = Air Temperature (Dry Bulb) in Centigrade (C) degrees
              RH = Relative Humidity in percent (%)
               B = intermediate value (no units) 
               D = Dewpoint in Centigrade (C) degrees



    Here is the same formula, but broken down into smaller pieces:

          L = ln(RH / 100)
          M = 17.27 * T
          N = 237.3 + T

          B = (L + (M / N)) / 17.27

          D = (237.3 * B) / (1 - B)

            where:
                    T = Air Temperature (Dry Bulb) in Centigrade (C) degrees
                   RH = Relative Humidity in percent (%)
                    L = intermediate value (no units) 
                    N = intermediate value (no units) 
                    M = intermediate value (no units) 
                    B = intermediate value (no units) 
                    D = Dewpoint in Centigrade (C) degrees

                    * = multiply by...
                   e^ = natural log (e to the power of...)




  Dewpoint calculated from:  Dry Bulb Temperature 
                             Wet Bulb Temperature
                             Barometric Pressure
------------------------------------------------------------------------------

     Es = 6.108 * (e^((17.27 * T) / (237.3 + T)))

     Ew = 6.108 * (e^((17.27 * W) / (237.3 + W)))

      E = Ew - (0.00066 * (1 + 0.00115 * W) * (T - W) * P)

     RH = 100 * (E / Es)

      B = (ln(E / 6.108)) / 17.27

      D = (237.3 * B) / (1 - B)


           where:
                    W = Wet Bulb Temperature in Centigrade (C) degrees
                    P = Barometric Pressure in kilopascals (kPa) 
                        (See site pressure table below)
                   Es = Saturation Vapor Pressure at Dry Bulb (mb)
                   Ew = Saturation Vapor Pressure at Wet Bulb (mb)
                    E = Actual Vapor Pressure (mb)
                    B = intermediate value (no units) 
                   RH = Relative Humidity in percent (%)
                    D = Dewpoint in Centigrade (C) degrees

                    * = multiply by...
                   e^ = natural log (e to the power of...)




   Barometric Pressure calculated from  Elevation
--------------------------------------------------------------------------

   P = 101.3 * ((( 293 - ( 0.0065 * Z )) / 293 ) ^ 5.26 )

     where: 
       E = Elevation in meters (m)
       P = Barometric Pressure in kilopascalss (kPa)
  
       * = multiply by...
       ^ = to the power of


 To Convert Feet (f) to Meters (m) :
-----------------------------------------------

   m = f * 0.3048

   where:
     m = elevation in meters (m)
     f = elevation in feet (f)



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   Source: 'Converting Humidity Expressions with Computers and Calculators
            Cooperative Extension Leaflet 21372.  
            University of California - Davis
            Author: R. Snyder and R. Snow

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