Thank you so much for this amazing and fascinating, well-written and presented overview of a question many of us are interested in. In "beauty" circles, water is considered essential for our skin. Where did you grow up in Cali, Eric? I'm from Redlands (and Hollywood). Now we have moved to SW Florida and when I hike (I use the term loosely) here, it's so different. I have a friend who is an outdoor enthusiast and recently-retired nurse and she talked to me about the "pee factor." In other words, I've been doing outdoor activities for years like you (no Half Dome tho, you lucky!!!) and I never understood why I'd be out x-amount of time and need to go behind a tree in some cases, and others, I could complete your Half Dome climb and nothing. She explained it was due to how much we were sweating - well duh! So here in Florida, the humidity affects the whole process much differently than drier climates like Cali. I will be out, and be sweating, but not feel particularly thirsty until I'm seriously dehydrated. Back home, I knew the signs much better and the feeling of thirst came on much more gradually. So, climate too -