Jack’s stories clear fog surrounding humidity

By Jack Williams ©2015

Since soon after becoming the founding USA TODAY Weather Page editor in 1982 Jack realized that many people were in a fog when they talk about topics involving water vapor in the air, that is humidity.

Below are links some some of Jack’s Capital Weather Gang stories that attempt the clear some of the mental fog surrounding humidity.


After the Weather Gang Blog published Jack’s explainer on Why dry air is heavier than humid air on Aug. 5, 2013, Jack responded to a reader’s question with the experiment described below:

A reader commented: “Suppose I run a dehumidifier in my basement. Does the dry air coming out of the dehumidifier tend to stay on the basement floor, or does it mix immediately with the more humid air around it?”

Jack performed a simple experiment to answer the question and replied with the results:

The Experiment 

The dehumidifier in my basement had been running at least 12 hours when the measurements were taken:

  • Floor in front of dehumidifier: 70.2 F RH 48% calculated Dew Point, 49.6
  • Shelf, 68 inches above dehumidifier: 74.5 F, RH 73.3 50% calculated DP 54.66

I used the air density calculator at: http://wahiduddin.net/calc/calc_da_rh.htm
Since It’s for pilots, I needed an altimeter setting for atmospheric pressure.
My home barometer read 1010.6 mb. My elevation is 250 ft AMSL
Using my home “station pressure” (the barometer’s actual reading) I used the calculator at
http://www.srh.noaa.gov/epz/?n=wxcalc altimeter setting to get the altimeter setting of 30.10 inches of mercury.
Using this information in the density calculator above I get the following:

  • Air density at the floor: 1.19 mg per meter cubed
  • Air density 68 inches higher 1.182 mg per meter cubed
  • Vapor pressure at floor 12.09 mb
  • Vapor pressure on shelf 14.56

Yes, the air in he room is stratified by temperature and moisture with more moisture in the air 68 inches above the floor than at the floor.

Instruments used:

  • Temperature and RH: EXTECH Big Digit Hygro-Thermometer.
  • Barometer: 3-inch brass aneroid barometer, souvenir from Germany. No manufacturer’s name on instrument. Readings below 973 mb marked “Sturm;” Readings above 1051 mb marked “Trocken.”

Relative humidity” is the most common measure of humidity you’re likely to hear. This measurement is confusing unless you know more about meteorology than most people. Jack explains this in his “Weather weenies prefer dew point.”

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