How Tempeature Effects Humidity

January 30, 2013

Why does air become more humid as it cools down and more dry as it heats up? Think of it this way, evaporate one cubic meter of water into the warm humid outdoor air inside a space the size of your back, side, and front yard and you will not detect much humidity in that vast amount of warm expanded air, cool that volume of air and it shrinks, let’s say you shrink it to the size of the interior of you home, obviously your home is smaller than the yard it sits on. Now you have less air volume but the volume of humidity ( evaporated water in the air ) remains the same and that humidity is squeezed closer together now, the result is more humidity in the air per given area and the relative humidity goes up. Cool that same air down even more, cool it down to such a degree that it occupies a space the size of one cubic meter.

Now that air that was the size of your front back and side yard and contained one cubic meter of water vapor now is one cubic meter of air and contains one cubic meter of water. Now the air is saturated with water, it is at 100% RH, we call this dew point and the water vapor molecules join together to form droplets of water we call condensation and mold now has water to grow.

There is a gradual progression of wetness in that ever changing volume of air and that gradual progression of wetness is measured in Relative Humidity percentage when testing your air. Around 60% to 65% Relative Humidity is all that is needed for mold to grow; we do not need 100% or condensation for it to grow.
Open doors and windows are often not a problem in northern or western regions but in hot humid Florida outdoor air often causes mold problems if the AC is on or off, but with the cold AC turned on condensation problems becomes even more of a problem in and around AC units and ducts.


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