We've finally had a burst of hot weather - 39 degrees last week and 36 yesterday (although a cold change came through overnight and I am currently freezing - I'll need arm warmers and a vest this morning!).
Riding when it's 39 isn't that bad if you're prepared for it. If you ask me, apart from drinking plenty of water before setting out, carrying enough for the trip and drinking some electrolytes, the main thing is to have a good level of fitness. The hills and the heat could easily kill someone who is badly out of shape. I'm now at a level where if I potter along easily, I don't sweat at all.
On hot days, I leave around sunrise so that I get to work whilst it is still relatively cool - I don't have a hard time cooling down with a shower. Riding home in the heat is unavoidable - you just have to suck it up. All I do is drop my speed by 20-25%. That causes a serious drop in the level of effort that I have to make, so I don't overcook myself. It's hard to rein in the speed when you're used to charging along a certain section at a certain pace - I really have to watch the speedo and think about it.
Yesterday was worse, even though the temp was lower at 36. There was a howling headwind blowing, which meant I had to work bloody hard to make it up some hills. That negated the "take it easy"philosophy. For instance, although I was in low gear going up the Anzac Bridge (doing a paltry 11km/h - almost standing still), I was still working like a maniac due to the wind. If I'd dropped my effort level, I would have come to a standstill and fallen off the bike. I had to pedal hard down the other side just to keep moving. I was thinking about hopping off and pushing it was that bad. It was seriously hard work, and I am still feeling it this morning.