Wx Savvy | Science

Dewpoint vs. Relative Humidity

Firstly, if you so desire you can find out more about the dewpoint in my savvy section on "What is the dewpoint?"

The higher the dewpoint temperature, the higher the moisture content of the air. So you will always know that it is more humid if the dewpoint rises. However, you cannot discern this from looking at changes in the relative humidity. I'll explain why in a moment, but it is for this reason that dewpoint is the better gauge for changes in humidity.

About Relative Humidity

Relative humidity (RH) is a ratio. It compares the amount of water vapor in the air to the amount of water vapor at which the air at this particular temperature and pressure will be saturated. So if it's 50% RH then the air is 50% saturated right now. The reason I said, "...at this particular temperature and pressure will be saturated." is because if you change the temperature or the pressure then you have changed the amount of water vapor needed for saturation. Typically it's the temperature that changes more easily then the pressure. Notice you haven't changed the actual water content of the air. And this is why it is called "relative" humidity. It is relative or dependent upon the temperature and pressure.

Dewpoint is a better absolute measure of the air's moisture content. Here's a good example of why:

Temperature	Dew Point	Relative Humidity
90	80	= 67%
100	80	= 47%
110	80	= 33%

From this table above, look at the relative humidity of 33%. Looks like a comfortable relative humidity, right?

Probably not. Sticky, sultry and oppressive are appropriate adjectives for 110 degree heat with an 80 degree dewpoint. But why is the relative humidity so low? Because relative to that hot temperature of 110 degrees, the air is no where near saturation. Remember, we would have to cool the air down to 80 degrees (with no pressure change) in order to saturate that airmass (that's the definition of dewpoint). That's a 30 degree cooling! If our air temperature is 90 degrees, now the air is 67% saturated and we only have to cool the air 10 degrees to reach saturation. What if the air temperature was 80 degrees and the dewpoint was 80 degrees? The air is 100% saturated, and it could be that you would be experiencing rain or fog.

About Dewpoint Temperature

Dewpoint temperature is a better "absolute" measure of moisture in the air. Why? Because it doesn't change when the air temperature changes, it only changes when the moisture content changes. (Assuming constant pressure). Dewpoint isn't an absolute measure though because it is dependent upon constant pressure. But pressure doesn't change very rapidly.

In general, I consider dew points at or above 70 degrees very uncomfortable and sultry. For most people dewpoints below 60 usually indicate a comfortable atmosphere.

Meteorologists consider dewpoints near 55 degrees and higher to be necessary for severe weather. That's just a guideline but it's usually true. It's because the higher moisture content in the air helps storms grow.