I have taken it upon myself to make it my life's work to get people to understand dew point rather than the more commonly used, but less useful to most, relative humidity. I have written before that all you need to know when listening to the weather report is that a dew point of 65 degrees or higher means high humidity and will be uncomfortable. The lower the dew point is, the dryer the air is. Today I am going to show you how useless it is for the average person to know what the relative humidity is.

The chart below charts out, from top to bottom, temperature, dew point and relative humidity. Relative humidity is a function of temperature, so notice that the temperature line (top) and relative humidity line (bottom) move in an almost perfect mirror pattern. But notice the dew point stays unchanged.


I grabbed this screen shot from the NOAA website a couple of years ago when we were going through a long hot and humid spell.

At night time (the lightly shaded area) the relative humidity shoots up to 89%, but during the day it drops to 43%. But it is not any less humid during the day, and it is just as uncomfortable as during the night. That is because relative humidity is not directly measuring how much actual moisture is in the air. It is telling you how much more moisture you can add to the air at a given temperature. Really useful to you, isn't it?

The dew point is a direct measure of how much moisture is in the air. It is not dependant on temperature and will not fluctuate when the temperature changes, only when the moisture content of the air changes.