Relative humidity
Relative humidity is defined as the amount of water vapor in a volume
of air expressed as a percentage of the maximum amount that the air
could hold at that temperature. The warmer the air, the more water
vapor it is capable of holding. Sudden cooling
without adequate dehumidification will precipitate moisture in the form
of dampness or even condensation.
The amount of moisture in the air
is important for a number of reasons. High humidity speeds up
deteriorative chemical reactions. With high humidity and temperature,
the growth of mold and mildew is
encouraged and the likelihood of insect infestation is increased. Low
relative humidity causes materials to become dry and brittle. Paper
that is dried out can break and crumble as it is handled and flexed.
Cyclical changes in temperature cause relative humidity to fluctuate.
Paper is a hygroscopic material, meaning it will absorb or emit moisture
depending on the ambient conditions, causing microscopic structural
changes that can be very damaging over time. | 
