SHORTCUT TO CALCULATING WETBULB
 
METEOROLOGIST JEFF HABY
A quick technique that many forecasters use to determine the
wetbulb temperature is called the "1/3 rule". The
technique is to first find the
dewpoint depression (temperature minus dewpoint). Then take this number and divide
by 3. Subtract this number from the temperature. You now have an approximation for the wetbulb temperature.
Here is an example: suppose the temperature is 42 degrees Fahrenheit with a dewpoint of 15 degrees Fahrenheit.
The dewpoint depression is 42  15 = 27. Now divide 27 by 3 = 9. Now subtract 9 from the original temperature
of 42. 42  9 = 33. If the temperature was 42 with a dewpoint of 15 and it started raining, the temperature
and dewpoint would wetbulb out to a chilly 33 degrees Fahrenheit. As dewpoint depression or temperature increase,
the evaporational potential increases.
This technique does not give the exact wet bulb temperature but it does give a pretty close approximation. Warmer
air will cool at a greater rate than colder air since more water vapor can evaporate into warm air. Evaporation
is a cooling process that absorbs
latent heat, therefore the more evaporation the more cooling. For temperatures between 30 and 60 degrees
F, the 1/3 rule works quite well. For warmer temperatures than 60, the cooling is between about 1/3 and 1/2 the
dewpoint depression.
It is important to know the wet bulb temperature in the
PBL in winter weather situations. Evaporative cooling can
change rain to (snow, sleet or freezing rain.) Example, suppose the temperature is 37 with a dewpoint of 18.
Using the 1/3 rule the temperature would cool to 31 F (now below freezing).
If the soil temperatures are warmer than the surface air temperature then the cooling will be less than the
1/3 rule. In this case, subtract 5 from the dewpoint depression before dividing by 3. This will give
a more realistic estimate of the actual wetbulb cooling that will occur.
If you want an exact wet bulb temperature you can use SkewT software, plot the sounding on a SkewT
yourself in the PBL and determine the wet bulb, or use charts in a meteorology book or computer program to determine
the wetbulb temperature.


