Chris_Dunn's Blog

 Here is a question from viewer Les Gomez of Littleton:

Hi Chris: Watch your newscast almost every night at about 9:16 PM.

 I have always wondered why is humidity expressed as "relative".

 What is it relative to? Thanks and regards.

Hi Les-     When we report “relative humidity” it is expressed as a percent of saturation of the air.  For example, 50% rh means the air contains about ½ of what it can hold at that temperature.  Because the total amount of moisture (the gas, water vapor) the air can hold varies with temperature, the humidity is relative to the air temperature.  A more absolute measurement of moisture in the air is dew point.  The dew point temperature is the temperature at which the air would be completely saturated.  The temperature/dew point spread can tell you how much moisture is in the air and the difference is the “relative” humidity.  Here is an example:  An 80 degree day in Denver in the summer feels pretty pleasant because the humidity is low…dew point of 35 degrees with an air temperature of 80 degrees = 12% relative humidity.  On the other hand, an 80 degree day in Miami, Florida in the summer (actually, it would rarely be that COOL in the summer in Miami) with a typical dew point temperature of 75 degrees = 81% relative humidity.  Same air temperature, but you have a long way to go in order to get that moisture in the air to condense in Denver than Miami.