Riding in the heat.
 

We are currently experiencing a heatwave in the southern part of the UK, temp up in 90's, and so far have not noticed a difference in my performance on the bike. Plenty of water with an isotonic additive seems to keep me hydrated, but I do not appear to sweat a great deal until I stop at the end of the ride. I suppose it could be the amount of liquid I take in, or the cycle specific clothing, or (Tongue in cheek this) the speed I go at keeps me constanly in a draft.
In the garden I wilt. I am dead- no energy to keep the garden tidy, no enthusiam to do the jobs that should be done, until it cools down in the evening. So- why is it that I can go out for 3 or 4 hours exercise and not feel the heat anymore than I would do on a winters day.

The draft evaporates the perspiration which cools you down. That's one of the things I love about cycling is it is about the most comfortable thing to do in the hot weather. The only way to tell it's really hot is the melting tar on the tires and the warm water in the bottles.

I suffer from the heat LOTS less cycling than I used to running or walking. The draft is the thing. I also hydrate regularly with both water and Sqwincher electrolyte juice. Currently 95 here every day with a 104 heat index.

You like to bike and you don't like to garden?

The high today in Phoenix, Arizona was 112*F. I didn't ride today but I rode for about an hour earlier this week when it only hit 109*. I ended that ride because of time available and not the conditions. In the interest of full disclosure the humidity today was all of 4% and that does make a big difference. Still, there is no denying it is BLOODY HOT! My observation is that the human body's cooling system is remarkably effective, particularly in a dry environment. I find that as long as I keep the water flowing in as fast as I'm sweating it out, I can tolerate the heat reasonably well. Avoiding sunburn is very important though. In more humid environments the evaporative cooling i not nearly as effective but the moving air produced by cycling will increase the amount of evaporation. Even though I've been drinking water while riding, it is not beyond my capacity to consume around 48 ounces of water at the end of a ride. But to reiterate, I try to keep the the water level stable rather than replenish it in one massive gulp at the end of the ride. BTW, I'm 49 years old.

Humdity/heat make a big difference.
Have cycled in Maryland on a century with 98 degrees and 98% humidity. Horrible!
Have ridden in 100+ degrees and single digit humidity in Tucson, AZ. Do-able!
'Warmest' I've commuted in: 117 degrees with 2% humidity, also in Tucson, AZ.
Eyeballs were burning behind my glasses, feet were hot from the heat bouncing off the pavement (probably in the neighborhood of 140+ degrees). Did not sweat. Was glad when I got home and hit the pool!

Like both but the best bit of gardening, is to plan all the jobs before starting them, while testing out to see if the hammock is still working properly

UK - Welcome to the club of heat sufferers. I guess global warming isn't just a cliche.
Your symptoms as described "In the garden I wilt. I am dead- no energy to keep the garden tidy, no enthusiam to do the jobs that should be done,... " are all but too familiar. Going out cycling on a summer midday here is more or less suicide. We have very little foliage (negev) and the heat hits you coming down and as well as going back up reflecting off the sand and bare soil. I try to limit my cycling to the early morn. 05:30 (sunrise) or late evenin 17:30 (sunsets here about 19:30).
The main concern here in addition to the danger of dehydration is exposure to intense radiation. Not long ago a couple of tourists hiking in the Dead Sea area died from dehydration. They were apparently unaware of the importance of hydrating the body on a regular basis even in the absence of thirst, once reaching a critical point, their bodies could not imbibe fluid, thus even when provided with fluid it did no good.

As a former Zonie, Arizonan, I advise drinking lots of water. Before you start out tank up as much as you can and drink more as you go along. You can't drink too much water.

And forget drinking beer, wine or any other booze while you are out in the heat. Save it for when you are in a nice cool place.

I had heat stroke once it was not fun at all, ended up spending a night in the hospital.

It was 100*F yesterday in Tracy Ca. My wife and I put 11 miles on our bikes. For me, 9 on a road bike and 2 on a MTB.

My brother-in-law says that the sweat is about 10 feet in back of you while you are moving down the road. It catches up at stop lights.

Joe