Always keep your instrument away from extremes of temperature and humidity. Always keep the instrument at room temperature whenever possible. Keep it out of direct sunlight and away from heaters. Avoid putting it in the trunk of a car or in a closed, unoccupied car. Heat can damage the varnish finish and cause serious cracks in the wood.
Hot temperatures cause the wood to expand, dry out, and crack.
Very hot temperatures will cause the glue which holds the instrument together to fail. Cold temperatures cause both the wood and varnish finish to contract and crack. Wooden instruments will dry out if the air lacks humidity. Instruments do best when the humidity in the air remains stable. Use a dampit in the instrument's case or run a humidifier in the room where the instrument is kept to prevent the instrument from drying out and cracking.