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 millibars (mb) 
                        (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...)


                 ELE(ft)   P(mb)                        ELE(ft)   P(mb)   
                 ======   ======                        ======   ======  
   Tucson         2340      932           Yuma Valley     114     1009  
   Yuma Mesa       191     1006           Safford        2955      912  
   Coolidge       1385      965           Maricopa       1185      972  
   Agulia         2150      939           Parker          307     1002  
   Bonita         4415      864           Citrus Farm    1335      967  
   Litchfield     1014      978           Phx Greenway   1315      968  
   Marana         1973      945           Yuma N.Gila     145     1008  
   Phx Encanto    1100      975           Eloy           1513      961  
   Dateland        533      995           Scottsdale     1540      960  
   Paloma          720      988           Mohave          480      996  
   Laveen         1020      978           Queen Creek    1420      964 
   Harquahala     1180      972
 



   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 millibars (mb)
  
       * = 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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