How do you calculate relative humidity?

How do you calculate relative humidity?

• For summer conditions, set the dew point temperature outside to 70F and the air temperature outside to 100F. The relative humidity is then about _____?

• Answer:

  As a pilot, I use a rule of thumb but since I am European, it is in degrees Celsius. I'll do the conversion: 100 F = 37 C and 70 F = 21 C. The difference between dew point and ambient temperatures is often called, the spread. Here it will be: 37 - 21 = 16 C. The rule of thumb is: Relative humidity = 100 - (spread * 20). In our case it would be: 100 - (16 * 5) = 100 - 80 = 20 percent. That is relatively dry air. To work the coefficient for Farenheit degrees, and with a spread of 30 F, we do: 100 - (30 * X) = 20 or X = (100 - 20) / 30 = 2.6 In Farenheit, the rule of thumb should then be: RH = 100 - (spread * 2.6) If you want a more accurate calculation, you should use a psychrometric chart. I am sure you can find one on the internet. EDITED: I co-posted with Cyswxman. His answer is different from mine and I am pretty sure his calculator is better than my rule of thumb. The point is: such calculations are never linear and I know that mine works best for higher levels of relative humidity, say between 60 and 100 percent. 37 percent relative humidity is something we never experience in Norway where I live. I guess you have to be in the center of the north American continent to experience that.