I have found, through careful research (the hair on my head and the horse hair on the bows that I use), that good bow hair days and bad bow hair correlate to good head hair days and bad head hair days. It is on the good bow hair days that I enjoy playing the most. On the bad bow hair days I tend to reach for the rosin and scrutinize everything in my bow-drawing anatomy (I also avoid looking in the mirror).
Too much humidity in the air causes the hair to relax, which makes the stick bear a different kind of burden from the burden it bears when there is a moderate amount of humidity in the air. When the air is too dry (like in some concert halls I know quite well), the hair bears more of a burden. The problem is compounded by the fact that sound travels more quickly through moist air that it does through dry air. Mathew Abraham gives an excellent explanation:
The density of dry air is more than that of moist air (Wonder why? Just answer me – which is denser – skimmed milk or fresh milk. The cream is lighter and when removed from milk, we get skimmed milk and therefore skimmed milk will be denser than the fresh one with cream content. Just like that the water vapor is lighter than dry air. When moisture is removed from air, its density increases). The speed of sound in a medium is inversely proportional to the square root of its density. Therefore, the speed of sound in moist air is more than that in dry air.Years ago, when I was deeply into the practice of making bread, I read an article in Gourmet magazine about the way humidity affects the wheat crop, and therefore affects the bread that is made from it. In order to have a consistent "product," bakers have to either have consistent ingredients or compensate for the inconsistencies that crop up from time to time. [Bad pun, but I'm keeping it in because it wasn't intentional.] String players, like bakers, have to come up with a consistent "product," regardless of what physical environment we happen to be in at the time.
Perhaps we should embrace the daily changes in the temperature and humidity (and they are daily these days) because they connect us more with the natural world and its inhabitants. And as "long hair" musicians that is something that we should strive to do.