Knowing the difference between dew point and relative humidity

With the recent hot and humid days, you've likely heard Meteorologists talk about the dew point temperature a lot.  Dew point temperatures typically over 60 to 65 degrees makes the air feel uncomfortable, or sticky.  Dew point temperatures over 70 degrees just feel down right miserable.

I received an email the other night wondering why we don't talk about the relative humidity that much.  After all, it does relate to humidity.  And that's a good question:  why do we stress so much about the dew point temperature?

To understand why, we first need to explain what the difference is between dew point and relative humidity.  Dew point is an absolute measure of how much water vapor is in the air.  The dew point temperature will always be lower, or equal, to the air temperature.  Relative humidity is a relative measure of how humid the air is, or, how close the air is to saturation.  Relative humidity is expressed as a percentage, and is the ratio of the pressure of water vapor in a parcel of air relative to the saturation pressure of water vapor in that same parcel of air at a specific temperature.  In simpler terms, a relative humidity close to, or at, 100 percent means the atmosphere is saturated and rain or clouds are likely. 

A dew point temperature at 65 degrees or higher means the air feels sticky, or humid.  A dew point temperature less than 65 degrees may not feel as humid and people are able to tolerate it a little more.  Relative humidity can be thought of as how close the air is to saturation.  For example, in the early morning when the temperature is close to the dew point temperature, the relative humidity will be near 100%.  But it may not feel humid outside because the dew point temperature is low.  During the day, the temperature will rise with the heating of the sun.  This causes a larger spread between the dew point and temperature, which can cause the relative humidity to decrease.  In very warm and humid conditions, the dew point temperature can often times rise to 70 degrees or higher.  No matter how warm the temperature gets, a dew point temperature of 70 degrees will always represent the same amount of water vapor in the air in absolute terms, but the relative humidity will change throughout the day.  This is why we use dew point temperature more so than relative humidity during the summer time because it is a better measure of how humid it feels.