– and what it means for guitars
Relative humidity is the ratio of the amount of water vapour in air to the maximum amount of water vapour that could be in the air if the vapour were at its saturation condition.
The relative humidity of a system is dependent not only on the temperature but also on the absolute pressure. Therefore, a change in relative humidity can be explained by a change in temperature, a change in pressure, or change in both of these.
Relative humidity will change inversely, albeit non-linearly, with the temperature (in other words, relative humidity falls as temperature rises). This is because the partial pressure of water increases with temperature – the operative principle behind everything from hair dryers to dehumidifiers.
Due to the increasing potential for a higher water vapor partial pressure at higher air temperatures, the water content of air at sea level can get as high as 3% by mass at 30 °C (86 °F) compared to no more than about 0.5% by mass at 0 °C (32 °F). This explains the low levels (in the absence of measures to add moisture) of humidity in heated structures during winter, indicated by dry skin, itchy eyes, and persistence of static electric charges. Even with saturation (100% humidity) outdoors, heating of infiltrated outside air that comes indoors raises its moisture capacity and is reflected in a lower indoor relative humidity and increased evaporation rates from moist surfaces indoors.
In other words, household heating reduces humidity of the air. Not only may your skin itch but woods – by losing some of their water content – may move and crack. This is the danger for all fine objects made of wood – from carvings to musical instruments. Items made of solid wood are more prone to cracking that those made of laminates because the latter has a strength derived from its structure – bonded layers of wood, each layer arranged with its grain at an angle to proximate layers.
Extracts from Wikipedia, the free encyclopedia with a few notes on the significance for the guitar from us.