As I speak its 40 degrees outside in Melbourne. 39 tomorrow. My trick for cycle commuting is to jump in the shower in my cycling clothes right before I go, then pour most of my water on my skin when I feel hot. The body can only use 300ml per hour anyway and I carry about a litre.
40 degress and you're drenching yourself down? That's a tactic reserved for when you're too dehydrated to sweat anymore!
And for those few American readers of this site... that 40 is in Celcius - so it's over 100 degrees.
Lessee, another attempt at command-line bc.....
~$ echo "(40 *9/5 +32)" | bc // Knew there hadda be a way.
HAAAAAAhahahahahahah!!!!! As soon as you get moving, 104 is fairly comfortable.
I find 105 to be a sort of tipping point. Anything beyond that gets pretty bad. YMMV.
Try 114 or so - uh, ~$ echo "((114-32) *5/9)" | bc
That's enough to cause little prickly sensations all over your body. I don't bother with the shower, because to walk outside in that covers me in sweat immediately. (Probably a conditioned response. Putting on my helmet, indoors inside the AC, will do the same.)
I agree that you cannot absorb water anywhere near fast enough to keep up with that kinda loss. 300ml/hour? I suck down well over a liter (2 extended-top bicycle bottles) in ten miles (maybe 45 minutes, in my old age), and have never lasted more than a couple of hours past 110. By that time we're looking at heavy dehydration, heat exhaustion and eventual heatstroke. On a Century ride (that's 100 _miles_)(in the spring, when it was only 100F) my pop had the foresight to weigh himself first. After riding all day and swilling water as fast as he could he'd lost ten pounds (about 5Kg).
And I imagine Melbourne is fairly near the coast? It was my understanding that temperatures in Oz (at least in the outback) pushed 120(F).
~$ echo "((120-32) *5/9)" | bc
Now that's HOT.
So if you wanna see what real, _deadly_ heat is, take a ride inland tomorrow.