Wet Bulb Calculation

Here is an example:
Suppose the temperature is 42 degrees Fahrenheit with a dewpoint of 15 degrees Fahrenheit. The dewpoint depression is 4215 = 27. Now divide 27 by 3 = 9. Now subtract 9 from the original temperature of 42. 42  9 = 33.
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.
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 evaporation potential increases. 
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. 