Where I live, temperatures and humidity levels change dramatically from summer and winter. Wooden instruments, like violins and violas, expand and contract based on temperature and humidity. This can cause a few issues like buzzing, slipped strings, and cracks or open seams. There are a few things you can do with your instruments to help minimize problems.
It is best to get a room humidifier and keep the humidity between 30-40% in that room. (I aim for 35%). A cool-mist humidifier is better for instruments (and humans) and will penetrate the wood better than a warm-mist humidifier. I do not recommend using a dampit because if you do not check your dampit it every day and the dampit is left in the instrument dry, it actually wicks moisture out of the instrument and can damage your violin or viola.
Do not leave your instrument in the cold.
Wood does not like extreme temperatures. Do not leave your instrument out in the car, and try not to store your instrument by a drafty window or near a heating element.
Let your instrument acclimate.
If you are going to be performing, plan to arrive at your destination in enough time for your instrument to acclimate to the temperature and humidity of the hall. Going from inside to outside and then back to inside makes the wood expand and contract, and it is best for your instrument to acclimate to the surroundings before you saw away at it with your bow. This also gives you some time to warm up your fingers and set up before your performance, so there are many benefits to getting to your venue early.
Use a silk bag or cover.
I do not have silk bags for my instrument, but I do have a silk top cover for my instruments. There are a few reasons to use silk. First, the bag or cover will protect your instrument from rosin, dust, and anything that is in your case. The second reason is the silk will protect your instrument from taking on or letting go of humidity too fast. The silk acts as a barrier and helps to slow down the process, giving your instrument a chance to adapt easier. A sudden chance of humidity is what causes cracks and seams to open, so the silk helps to assure a more comfortable rate of change for your instrument.