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Bow hair comes from the tails of horses in cold climates such as Siberia, Mongolia, and Canada. The cold climate causes the horses to produce hair that is thicker and stronger than that produced by horses in warmer climates.
Temperature and humidity influence how much you need to tighten your bow. Hot, muggy weather causes the hair to stretch. We often need to tighten the tension more frequently in the summer months. In cool dry weather, the hair dries and contracts. Sometimes when we open our case the stick of the bow is straighter than how we left it when we were done playing as the hair length has contracted in our dry homes. Always loosen the tension of the hair on your bow when you are done playing.
Emily Sunderman 