Humidity Explained

Each month we publish a newsletter titled IAQ Website of the Month.  The excerpt below was originally published in the April 2009 newsletter.

Humidity is pretty simple, right? Wrong. There are several misconceptions that this month’s featured website tackles head on.

It is critical for indoor environmental professionals to understand humidity. It affects occupant comfort and plays a critical role in the growth of mold and dust mites. Central to an understanding of humidity are the concepts of relative humidity and dew point.

This month’s website was developed by Steve Horstmeyer, a meteorologist in Cincinnati (pictured). I think he does a wonderful job of explaining the complexities of humidity in layman’s terms.

Have you ever talked about air “holding” water vapor? You’ll soon learn to strike that from your vocabulary. Can you explain why evaporation and condensation occurs? I’m guessing you’ll change your answer after reading through the website.

To visit this month’s featured website, click Humidity by Steve Horstmeyer. A special thanks to one of our students, Eric Barker, for submitting this website.

To subscribe to this newsletter click the following link: IAQ Website of the Month.

Humidity illiteracy

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 []

Introduction to Psychrometrics, Part 3 of 3

In the first two installments of “Introduction to Psychrometrics” I covered concepts such as air, evaporation, temperature, condensation and dew point. I strongly encourage you to read Part 1 and Part 2 before reading this final installment where I’ll be explaining relative humidity, humidity ratio and a few other concepts.

The amount of humidity in the air will affect the indoor air quality, therefore it is important to measure it. Unfortunately, there are four common terms used to quantify humidity: relative humidity, humidity ratio, absolute humidity, and specific humidity. I’ll cover all four concepts in this post.

As a quick refresher, humidity is a measure of the water molecules in the air that have escaped the surface of liquid water. I’ll be using the term “water vapor” to describe these molecules. Water vapor is the result of evaporation (see the word “vapor” hidden in there?).

Relative humidity, or “RH”, is the most commonly used expression for humidity. It also happens to be the least understood.   Relative humidity is the ratio of water vapor in the air compared to fully saturated air at the same temperature.  In other words, there is a certain amount of kinetic energy in a system to free water molecules.  RH looks at how much of the system’s kinetic energy has been used to free molecules.  When I use the term “system” I am referring to the air + any liquid water that may be present.

If a room has a relative humidity of 40%, it still has a lot of unused energy (60%).  Put a cold glass of water in that room and the kinetic energy in all gas molecules (nitrogen, oxygen, water vapor etc.) will transfer heat to the cold water.  When the water molecules heat up, that increases their kinetic energy and ability to escape the liquid surface.

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Introduction to Psychrometrics, Part 2 of 3

In my first installment on psychrometrics, I covered the basics of air, humidity and evaporation. Here I’ll cover temperature, kinetic energy, attractive forces and condensation.  If you haven’t already, I strongly suggest you read Part 1 first.

We’re all familiar with the general concept of temperature. Temperature ends up being quite complex if you dig deeper (and deeper we shall dig).  Technically, temperature is related to the average energy of motion, known as kinetic energy. To use our illustration from the last blog post, the faster the billiard balls move on the table, the greater the temperature. Continue reading