Enthalpy is the problem
The combination of temperature and humidity is called enthalpy. As the air in your home or building is heated (by the furnace) it's ability to absorb moisture increases (that's called the saturation level). As that happens, everything that holds moisture is subject to giving up some of the moisture. Like all of your mucus membranes (lips, nasal tissues, etc). Wood absorbs a lot of moisture and, consequently, gives off a lot of moisture. As a guitar is giving off moisture the wood actually shrinks which makes it go sharp. It also affects the neck angle somewhat which means that you may have to adjust the truss rod. As a guitar absorbs moisture in the spring (when humidity levels go up and the furnace goes off) you will find that you have to adjust the neck again. The solution is to keep your guitar in the case along with a good case humidifier. Keep a room type humidifier in the place you store guitars. This affects all wood so pianos are very much affected by moisture and humidity. You can buy inexpensive "jar type" case humidifiers for a few bucks and they work well. If you don't have that, cut a sponge up and soak it in water, shake off the excess and put it in the case.OK, I'm an engineer geek.
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