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 and Relative Humidity 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)

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

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

RH = 100 * (E / Es)

where:
W = Wet Bulb Temperature in Centigrade (C) degrees
P = Barometric Pressure in millibars (mb)
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 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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