Humidity Illiteracy

Jan 5, 2011

An open letter to the editor of Indoor Environment Connections:

Carl Grimes denounced illiteracy and innumeracy with great elocution in his article “A Serious Problem- Innumeracy” (Volume 11, Issue 11). My good friend Mr. Grimes would have made a more convincing point had he not exhibited illiteracy and innumeracy himself when describing absolute humidity. What he described in his article as absolute humidity (“grains of moisture per pound of dry air”) is actually humidity ratio. He should have expressed absolute humidity as grains of moisture per cubic foot of air (or some other volume). ASHRAE defines absolute humidity as the ratio of the mass of water vapor to total volume of the sample1.

I normally overlook this common misunderstanding, but when considering the article’s subject matter, it was too ironic to ignore.

Ian Cull, the Indoor Air Nerd

ps. Carl encouraged me to write this!

  1. 2009 ASHRAE Handbook-Fundamentals. American Society of Heating, Refrigerating and Air-Conditioning Engineers, page 1.2