During this winter season, it is very important to maintain the humidity level. Many cracked bridges and cracks in tops or backs can be prevented by taking good care of the guitar in winter.
Above all, purchase a hygrometer to measure the humidity in a room. This should be between 45% and 55%. With frost, the humidity can easily drop to 30%, and this often happens quite quickly. Especially when the heating is on high, the humidity in the room drops. Signs on the guitar are: concaving (or 'sinking') of a top or back and sharp fret ends. The wood of the fretboard shrinks, causing a fret end to protrude. Usually, it's not long before a loud crack is heard and a crack in the top or back is discovered.
In my early days as a builder, I had built a pineapple ukulele. At the time, I wasn't monitoring the humidity in the workshop, and as a result, the instrument turned out to have been built at 70% humidity. I proudly took the instrument inside, as it was my first self-built ukulele. I found a nice spot on the wall, and you guessed it... the humidity in the living room was different and below 40%. The result the next morning? Cracks all over the instrument that were irreparable. I was extremely disappointed and started delving into the theory of humidity. Since then, instruments are built under the correct humidity and temperature, and I prevent a lot of misery. 
The easiest way is to put a humidifier in a room, and these are quite capable of keeping the room at 50%. I have a humidifier here that switches itself off at a certain humidity level. Furthermore, you can store the guitar in its case with a good humidifier. This way, you ensure that your instrument gets through the winter months in good condition. Ramon Riemersma
Luthier
