Wetbulb Temperature and Dewpoint Temperature from Air Temperature
and Relative Humidity
Note: This calculation is very complex and requires a knowledge of algebra.
From the user, an air temperature ,
a relative humidity ,
and a station pressure
are given. The air temperature must be converted to units of degrees Celsius
,
and the station pressure must be converted to units of millibars
or hectoPascals .
To see how to convert temperatures and pressures, see the links below:
tempConvert.php
pressureConversion.php
Then, the saturated vapor pressure
can be calculated using the formula from the link below:
vaporPressure.php
Next, the equation for calculating relative humidity:
can be solved for the dewpoint temperature:
The dewpoint temperature will be in units of degrees Celsius .
To see how to convert temperatures, see the link above.
Next, a wetbulb temperature
must be calculated. The best way to do this is by using a SkewT diagram which
is used by the National Weather Service and other meteorologists for determining
the current state of the atmosphere. A blank SkewT diagram can be found here
at this link:
http://meteora.ucsd.edu/wx_pages/stuff/Blank_SkewT.pdf
For information on how to read and understand a SkewT diagram, see the link
below:
http://weather.unisys.com/upper_air/skew/details.php
For finding the wetbulb temperature, first find the elevation of your location.
Next, at the elevation of your location, plot the air temperature (in degrees
Celsius) and the dewpoint temperature on the chart. Take the air temperature
up the dry adiabat line and the dewpoint temperature up the theta line until
they meet. At the point where they meet, come back down the moist (or wet) adiabat
to the elevation of your station. This will be the wetbulb temperature. For
the Weather Calculator, a process of error checking is used to give an approximate
wetbulb temperature. Information on how the Weather Calculator approximates
the wetbulb temperature can found at this link here:
http://meted.ucar.edu/awips/validate/wetblb.htm
