Too little moisture in the air for too long and your guitar could crack; too much moisture and it can swell and become hard to play. The measure of the moisture in the air relative to temperature is called Relative Humidity (RH) and this is expressed as a percentage of the airs ability to hold water.
The best way to control your guitars environment is to purchase a hygrometer that has an external gauge and display plus a wireless secondary instrument that you can insert into your guitar case. I have been using one by Acurite for over a year now and I have found it reliable enough for the purpose. There are few places in SouthA frica where the RH is extreamly low or excessively high for long periods of time but anything above 70% or below 30% for more than a few days is problamatic. However, if the change is slow then the guitar would probably handle these extremes reasonably well.
The simpist and cheepest way of controling the RH is to keep your guitar in its case whenver you are not actually palying it. In addition to this, make a simply humidifyer and place it in the guitar case just under the headstock right up against the small accessories compartment. Here is how you make the humdiifyer:
1. Take an empty plastic butter/buttro container about 150mm x 100 x 50 and drill or cut a number of holes in its lid.
2. Cut a piece of sponge to size (the type you use for washing your car is ideal) and place it inside the container, ensuring that there is a gap of about 10mm between the sponge and the lid.
3. Dampen the sponge well but not excessively with one water and one part propylene glycol. However, when you top up you usualy need to only add water as the propylene glycol will remain in the sponge for quite some time. I have forund that in typical South African conditions 50/50 propylene glycol to water works fine and keeps the RH at about 45-50%.
As the RH drops the sponge will release moisture into the air within the guitar case. However, what is not often mentioned, is that it will also remove excess moisture when the RH is high. By mixing propylene glycol to the water you can control the maximum amount of moisture the humidifyer will release. You will have to experiment a little with just how much propylene glycol to use to get the stabilty point at arround 45% to 50 % RH. You can order the propylene glycol from http://e-liquid-concentrates.co.za/ Propylene glycol is both an antibacterial and antifungal agent preventing mold from growing in the container.
With this simple humidifyer you should be able to keep your guitar all year round at a RH level it is happy with and avoid any of the nasty problems that could otherwise occur.