Wet and Dry Bulb

A JavaScript routine that calculates the relative humidity given the wet and dry bulb temperatures.

Enter the dry bulb temperature ( Tdry ) and wet bulb temperature ( Twet) in degrees Celsius, then click on the Calculate RH button



How the calculation works:

1. The pressure is assumed to be 101.3 kPa (kiloPascals).

2. A conversion factor is calculated: A = 0.00066 (1.0 + 0.00115 Twb)

3. The saturation vapor pressure is calculated at Twb: eswb = e[(16.78 Twb - 116.9)/(Twb + 237.3)]

4. The water vapor pressure is calculated: ed = eswb - A P (Tdb - Twb)

5. The saturation vapor pressure is calculated at Tdb: esdb = e[(16.78 Tdb - 116.9)/(Tdb + 237.3)]

6. The relative humidity is then calculated: RH = 100 ed/esdb

This algorithm has been lifted from a U.S. Water Conservation Laboratory page which no longer exists. 

Here's another version.

Debut: December 24, 1997. Revision 4. November 12, 2006.
Index of other pages. Please send comments to Jeffrey Clymer.