Moisture Content and Humidity 

Because wood is organic and porous, it is affected dimensionally by changes in the amount of moisture it contains. Solid-wood instruments are more susceptible to the effects of changes in humidity than laminated instruments are, and therefore require ongoing attention to their condition. 

Protection From Temperature and Humidity 

The greatest natural threats to a  solid wood instrument are extremes of and rapid changes in temperature and humidity. High humidity can soften the glues used in building an instrument, and can cause the top and back  to expand and rise—raising string action too high.  

Or, if you live in a drier, low-humidity climate (hot or cold) or travel with your guitar to one, evaporation of moisture from the wood can cause shrinking and cracking. This can happen no matter how old the wood is, and string action can become too low if the top and back shrink enough. 

The ideal temperature for solid-wood instruments is in fact normal room temperature, which is about 70 degrees. 

The ideal humidity is about 40 to 50 percent. In winter, the heating systems used in many homes can drive temperatures up and humidity levels dangerously low . A good way to protect your Ukulele from drying out is to use a room humidifier. 

When an instrument is not in use, it should be kept it in its case along with a small humidifier (of which many are available).  

Do not leave an instrument out for long periods near a heating vent, radiator or other heat source; do not leave it exposed to direct sunlight; and do not leave it in a car for long periods. The general rule is to keep it safe from excessive heat, cold, humidity and dryness.