One of the main precautions we have to consider with our clarinet is to avoid undesirable splits, basically in wood clarinets which are more prone to cracking in winter. Why this happens? According to the Yamaha musical instrument maintenance guide, the reason depends on the room temperature, where the clarinet may be fairly cold in winter before it is played. Therefore, when the clarinetist blows into the instrument, the inside of the body of the clarinet will be warmed up at the same as his body temperature, around 36ºC and 37ºC (around 97°F). Consequently, the wood will try to expand outwards and since the outside of the instrument is already constricted by the low external temperature, the clarinet may be unable to withstand this temperature difference and crack.
So, in order to protect the clarinet from cracking, we must blow air through the instrument prior to playing to warm it up, as well as taking the clarinet out of its case, warm it up by putting it inside a coat or pressing it against our body. Keep in mind the upper body is most prone to cracking, which is most easily warmed by the breath of the clarinetist.