Wet-Bulb Temperature and Relative Humidity from Air Temperature
and Dewpoint Temperature
From the user, an air temperature ,
a dewpoint temperature ,
and a station pressure are
given. The temperatures must be converted to units of degrees Celsius ,
and the station pressure must be converted to units of millibars
and hectoPascals .
To see how to convert temperatures and pressures, see the links below:
Then, the saturation vapor pressure
and the actual vapor pressure
can be calculated.
To see how to calculate the vapor pressure, see the link below:
Next, the relative humidity can be calculated by using the vapor pressures.
Next, a wet-bulb temperature
must be calculated. The best way to do this is by using a Skew-T diagram which
is used by the National Weather Service and other meteorologists for determining
the current state of the atmosphere. A blank Skew-T diagram can be found here
at this link:
For information on how to read and understand a Skew-T diagram, see the link
For finding the wet-bulb 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 wet-bulb temperature. For
the Weather Calculator, a process of error checking is used to give an approximate
wet-bulb temperature. Information on how the Weather Calculatorapproximates
the wet-bulb temperature can found at this link here: