It is very dry in Minnesota during the winter months. This can be hard on string instruments - your students have noticed this in class with more loose strings, more tuning time needed during lessons, and in their at-home practice sometimes not sounding quite as good as they know it should!
As the humidity drops during winter, the moisture level in the wood of the instruments decreases, and the instrument 'shrinks' just a bit. The pegs don't fit quite as snugly into the holes in the peg box, and they can slip more frequently during winter (causing loose strings occasionally). Lack of humidity can also cause the top/bottom of the instrument to separate from the sides (this can easily be glued back together by a repair store), or in an extreme cause, cracks can occur in the wood of the instrument.
It can be helpful to humidify the instrument during the winter months. You can do this in a couple of ways.
1) Create your own humidifier: Use a small bottle (an old film canister or similar container works well) and poke several holes in the side. Insert a sponge into the container, and add some water to wet it down. This container is then kept in the case of the instrument. The sponge will need to be dampened each time your student practices in order to maintain adequate humidity.
2) Purchase a Damp-it. This is a small green tube with holes, and a sponge inside. The tube gets soaked in water, the outside is dried off, and the damp-it is inserted into the f-hole of the instrument. These can be purchased at a music store (visit www.eckroth.com and request store delivery in the on-line ordering), or request an order form from your orchestra teacher. Again, it is important to soak the damp-it at each practice session in order to maintain adequate humidity. Click HERE for a short video about how to use a Damp-it.
3) Other humidification products are available. You can contact your string rental store, or a local music store, to ask about options. I personally use a Planet Waves humidifier for my home instrument. This is a small hard plastic case with holes, and a sponge inside, attaches to the inside of the instrument case. I also have a long, hard plastic tube that fits into one of the bow holders on the inside of the cases, and has a sponge inside. I soak both of these when I practice, and then return them to the case at the end of a practice session.
String instruments are best kept away from heat sources (heat registers, space heaters, warm air drafts), as the warm air in winter months tends to be drying. I generally humidify my instrument case between November-April, and we will start to notice the humidity rising, and the instruments stabilizing.