jdrakeh
Front Range Warlock
Paper is a such a porous substrate that it reacts pretty quickly to humidity so it can get this waviness pretty quickly
I've seen it happen in less than eight hours to an entire five shelf bookcase full of. . . er. . . books. . . due to an overnight change in temperature and humidity. The worst my own books have ever experienced the phenomena was when I moved from Topeka, KS to Ridgecrest, CA back in 2002. That messed all of my books up for a good week and a half.