Dewpoint calculated from: Dry Bulb Temperature
Relative Humidity
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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
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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
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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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