Dewpoint 101

Bob Rose, chief meteorologist for the Lower Colorado River Authority, says we need to know our dewpoint number to understand humidity.

"Basically, anytime the dewpoint gets in the 60s, or higher, the air is going to feel pretty humid.  And the closer the air temperature and dewpoint readings are, the more humid the air will be."

Bob and Troy Kimmel, a lecturer in meteorology at the Univervsity of Texas, aren’t pleased that Houston’s KPRC-TV forecasters have decided to stop mentioning the dewpoint on the air–although the number still is available on the station’s web site. Troy, who forecasts for KVET/KASE/KMFK radio in Austin, as well as teaching meteorology, offers this lesson on dewpoint.

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To teach a little (after all that’s what I do!!), dewpoint temperature is the temperature you would have to cool the air to, at a constant pressure and water vapor content, in order for that air to become saturated (in other words for the relative humidity to reach 100% in the layer of air being monitored). In our atmosphere, the air temperature can never cool below the air’s dewpoint temperature.

Dewpoint temperature is a accurate measurement of the amount of moisture in the air. Relative humidity, on the other hand, is the ratio of the amount of moisture in the air compared to how much moisture could coexist in the air at that given temperature.The closer the air temperature and the dewpoint temperature are together, the higher the relative humidity.

Relative humidity only tells you how close to saturation you are, not how much moisture is present in the monitored layer of air. Relative humidity is always expressed as a percentage (%) and cannot be computed exactly without the use of psychrometric calculators and tables (although, again, it can be estimated by looking to see how close or far apart the air temperature and dewpoint temperature is).

A couple of other things to remember when discussing humidity:

– A 100% relative humidity does not necessarily mean rain. It simply
means the atmosphere is saturated. Clouds may form, but most clouds do not produce precipitation.

– The colder the air temperature, the less atmospheric moisture that can
coexist in that air. The warmer the air, the greater the amount of atmospheric moisture can coexist in the air.

Here’s a quick brain teaser that I use in my classes. Review these examples:

Example #1:  Air Temperature…. 90 F     Dewpoint Temperature…. 72 F
Example #2:  Air Temperature…. 18 F     Dewpoint Temperature…. 17 F
Example #3:  Air Temperature…. 65 F     Dewpoint Temperature…. 30 F
Example #4:  Air Temperature…. 40 F     Dewpoint Temperature…. 35 F
Example #5:  Air Temperature…. 100 F   Dewpoint Temperature…. 15 F

In which of these examples would the amount of moisture be greatest in
the part of the atmosphere being monitored?

In which of these examples would the amount of moisture be lowest in the
part of the atmosphere being monitored?

In which of these examples would the relative humidity be the highest?

In which of these examples would the relative humidity be the least?"

(Answers below – don’t scroll below until you want to see answers!!)

Troy M. Kimmel, Jr.
Lecturer and Manager, Weather and Climate Resource Center,
     Department of Geography and the Environment,
     University of Texas at Austin

Answers:

Moisture greatest? Example #1 (highest dewpoint temperature)

Moisture lowest? Example #5 (lowest dewpoint temperature)

Relative humidity highest? Example #2 (the air temperature / dewpoint temperature spread is the least; even though the relative humidity is highest, the amount of moisture coexisting in the air is relative low because the air temperature is low)

Relative humidity lowest? Example #5 (the air temperature / dewpoint temperature spread is greatest)