A general rule of thumb is that if you are uncomfortable, so is your instrument. So room temperature is best. If you must take your instrument out in extreme heat or cold, the best thing you can do is introduce it slowly. Moving from hot to cold very quickly can be more damaging than prolonged exposure. Heat is dangerous because the wood will expand. Also, since the violin is made of two different woods, maple and spruce, they expand and contract different amounts. In the cold, there isn’t much humidity in the air, so it can dry out your instrument. Either way it can cause cracks, and the smallest of cracks can cost hundreds of dollars to repair.