Dehumidification

How comfortable you feel depends on the amount of moisture in the air, or humidity.  When humidity levels are high, the air is holding lots of water vapor, which means the moisture on you skin does not evaporate as well as when the air is dry.  This makes you feel warmer than you would feel if the air was less humid and evaporation was taking place freely.  Dehumidifiers remove moisture from the air making it more comfortable. For some, drier air is beneficial to their health, as demonstrated by the number of people retiring to the desert southwest.  In addition, drier air reduces the occurrence of moisture related problems in the home, like mold, mildew and rot.

Dehumidifying homes is done in two ways.  One is simply by running the air conditioner, and the other is by using a dehumidifier, a device designed just to dehumidify air without cooling it.  They both use essentially the same process.  While the home air conditioner's primary job is cooling the air, it dehumidifies it too when warm moist air passes over the unit's cooling coil.  As the air cools, it can no longer hold as much moisture, so the moisture condenses on the coil and collects in a drip pan.  Cooler, drier air is then circulated to the home. 

When a home's air conditioner isn't enough, like in damp basements, a dehumidifier may be necessary.  Dehumidifiers operate using the same basic principal of air giving up its moisture as it passes over a cool coil.  They just go the next step to add some heat back to the air before releasing it to the space.