As we enter into the hottest months of the year, keep in mind that your instrument reacts to temperature changes just as humans do. Just as you wouldn't leave your dog or your child in a hot car, you should never leave your violin in one either. It is especially bad for an instrument that has been kept in a hot car to be brought directly into a cool air conditioned environment. Like people and animals, instruments are very sensitive to temperature changes. They can adapt to temperature changes over time but going from one extreme temperature to another is stressful for the instrument and can cause damage.
Heat can soften the varnish on an instrument and extreme humidity can weaken the glue joints and soften the wood causing deformation. Bows can easily bend or snap if they go from a hot car trunk to a cool air conditioned room.
Damage to an instrument can be expensive to repair but is easily avoidable. One way to keep an eye on the humidity level surrounding your instrument is to keep a hygrometer in your case. Hygrometers are especially useful in the winter when dryness and a complete lack of humidity can lead to cracks in an instrument. Overall the best way to prevent damage is to keep your instrument away from extreme hot or cold and if you do need to transport your instrument in a car, make sure you don't put in in the trunk or in direct sunlight.