It is not so much about temperature as it is about humidity levels. If humidity in the area you are in gets below 35%, that is an area where you might want to consider either a dampit, or a humidifier in your home. This type of humidity implies dry air and typically happens in colder weather. It also is dangerous to have your violin in extreme heat, but I would say this is above 90 degrees. This can happen if you leave the violin in a car in the hot summer time, where things bad can happen. For both situations, let me give you the extremes:
Very Dry – Cracking, open seams
Very Hot – Potential separation of wood (worst case scenario neck comes off the body of the instrument–I’ve seen that happen twice)
In hot weather a mild occurrence is strings going out of tune.
Hopefully this post gives you enough fears to take humidity seriously! 🙂