I read an article on the physics of violins a while back (I can't read it now without paying significant money ($80 or so). I think someone else at work had a subscription to that site and I piggybacked on theirs without knowing it. If it wasn't for that, I'd post the URL.)

The bow hair and string interact via rosin. I don't know for certain, but I suspect there has to be some rosin on the strings as well as the hair. There certainly will be rosin on the strings in short order, so I think it's safe to say that it's a rosin<->rosin interaction.

The action of the hair(rosin) on the string is what they called stick-slip motion. The bow (which if you're being technical and picky about it is actually the rosin on the hair) catches the string and pulls it away from current spot. As the pressure builds, eventually something gives, and the string starts sliding along the bow. It works very much like an ice skater - it melts the very top layer of rosin (we're talking 1/10s or maybe even 1/100s of a millimeter here) and skates along it, losing energy as it vibrates. When it has lost enough, the string (which is moving this whole time) catches it again, and the cycles starts over.

Combine that with another article I read that found that humidity had rather little effect on rosin - temperature had a much bigger effect, and the difference between light and dark rosins is the temperature they melt at. (Light ones melt at a higher temperature, typically.)

Since you have to melt the rosin (even if it's only a microscopically thin layer) for the process to work right, temperature probably makes a much bigger difference than humidity does.  My house is fairly climate controlled. The temperature is within a few degrees of the same summer and winter, and the humidity maxes out at 70% in the summer (because of the air-conditioning) and 50%-70% in the winter. (I got a whole-house humidifier to make sure it stayed up. There's nothing to keep it from going higher in the winter, but I've never seen it go higher than that.)

I mostly use the Holstein rosin these days, and have not noticed any big difference with the seasons.  Of course, I also haven't seen any big difference in performance with various kinds of rosin (not that I've tried that many).  I like the Holstein stuff because it produces less dust than most of the others, but all of them that I've tried (including some really cheap ones) make the strings move. That might be because of the conditions I play in. If I played outside frequently (or let the inside of my house swing a lot more between seasons), I might see very different results.