We often hear the term “relative humidity”, because the amount of moisture or water vapor a given volume of air can hold varies with the temperature of that air. As air is heated, its volume expands and its water vapor content, relative to the volume, decreases. Relative humidity increases as the air is cooled and the volume contracts.

 

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A relative humidity reading of 100 percent means that at that temperature the air is holding all the water vapor it can and has reached its dew point. The slightest bit of additional cooling will cause the air to release vapor in the form of moisture. That’s why dew or frost forms at times on lawns, cars and other cool surfaces outdoors at night. The same thing can happen indoors, particularly on windows and behind walls.

The atmosphere’s ability to hold moisture approximately doubles with every 18^oF / 10^oC increase in temperature. That’s why a 100-percent relative humidity reading in winter is a lot dryer than 40 percent on a hot summer day.