except for the sweat sounds like heat stroke ... no? ever pour water on your head? sounds like a really tough ride! I don't know how people can do it - been watching the pros in the TDF and I'm just amazed.
btw: some news program had a PSA about how much water we can lose every 20 min in very hot weather and the guy held up a 1 liter bottle of water. maybe those are the stats he used but not far from it