Overview

Most people recognize that water evaporates into air and when this occurs, the atmosphere becomes a mixture of gases and water vapor.

The water vapor in air is known as humidity.

Controlling humidity is not only important from the standpoint of comfort but also significant when it comes to maintaining healthy indoor environments.

Relative Humidity
The amount of moisture in air can be expressed in a number of ways but the most common reference is relative humidity.  This value, expressed as a percentage, represents the amount of moisture in the air relative to the amount of moisture air could hold at that temperature.

In other words, air at 70 degrees Fahrenheit (70^oF) and 50% relative humidity (50% RH) is holding half of what it could hold when completely saturated (100% RH) as shown on the right.

Therefore, the relative humidity at a given temperature will range from 0% (dry air) to 100% (fully saturated air).  (When you exceed 100%, moisture starts falling out of the air and it rains.)

Air/Moisture Relationships
The properties of air and moisture have been studied for decades and are known as psychrometrics.  Determining these properties involves complex equations or the use of psychrometric charts or calculators to approximate the conditions. [Try the online calculator]

At a given barometric pressure, temperature is the key factor regarding how much moisture can be retained.

What is important to remember?  At the same relative humidity, warm air holds more moisture than cold air (see illustration at right).

Illustration
(Amount of Water per Pound of Air at Sea Level)

90^oF/50%RH = 107 grains

70^oF/50%RH = 55 grains

50^oF/50%RH = 27 grains

30^oF/50%RH = 12 grains

(One pound of water = 7,000 grains)

Cooling air that is 90^oF/20% RH causes the relative humidity to increase until it reaches 100%; the temperature known as the dew point (43.6^oF in this example).  At the dew point, moisture begins to condense out of the air and is transformed from vapor to liquid.

Most everyone has witnessed bathroom mirrors 'fogging' during a hot shower or iced-drink glasses 'sweating' on the outside.  The cool surfaces are simply condensing moisture out of humid air.

This aspect of psychrometrics is very important when it comes to analyzing indoor conditions as temperatures and humidity levels within our environments are very dynamic.

Changes in Relative Humidity
Mother nature establishes our humidity and depending on geography, weather and seasons; outdoor levels are constantly changing.

Outdoor air creates the baseline for indoor humidity levels and we typically heat or cool this air to maintain our desired temperature range.  In the process of changing the temperature of air, we are altering the indoor relative humidity as explained above.  

Numerous activities add moisture to indoor air: occupancy (people exhale moisture and perspire), cooking, cleaning, etc.  In addition, we can intentionally increase moisture levels using humidifiers.  (Humidification is rarely required in the Dallas/Ft Worth area due to the natural ambient levels.)

Most refrigerant-type air conditioners, by design, remove moisture from the air during the cooling cycle through condensation at the evaporator coil.  This moisture is removed from the interior using a condensate drainage network.  In certain applications, dehumidifiers may be necessary if indoor relative humidity levels get too high.

Ave Annual Outdoor Humidity
Dallas/Fort Worth Area
(National Climatic Data Center)

Morning Average: 81%RH
Afternoon Average: 59%RH

Click HERE to see Chart

What should it be?
Studies of personal comfort have shown that relative humidity ranges between 30% and 65-70% can be considered 'comfortable' depending on activity.

However, from the standpoint of indoor air quality, upper ranges should be maintained below 50% (dust mite populations increase rapidly at relative humidity levels above 50% and fungal amplification occurs above 65%).  

Buy a Hygrometer and keep track of your indoor humidity levels.  These instruments are relatively inexpensive and can generally be purchased online or in many hardware or discount stores.

Target Indoor Humidity Conditions

30 - 50% RH

If it's raining, is the air always at 100% relative humidity?

Not necessarily.  Relative humidity conditions near ground level can be much less than at cloud level; rain is leaving air that is saturated (100%RH), but is falling through air which may be at lower RH levels.

[Back]