I like riding when it's hot out, but in my area it's never very humid, so sweat evaporates easily. If we had the humidity typical of the South, that would be another story!

I'm with lotek, I try to take both a Camelbak and bottles. I remember one hot trip from Spokane to Pullman in which I calculated my total water consumption... I drank 11 pounds of water in 75 miles, or 5kg in 120km. That's 15% of my body weight edit: oops, can't do my math... 7%, and I still weighed less when I arrived.

So drink like a fish, and know where to get more water If the humidity is high and your sweat isn't evaporating easily, don't overdo things.