Why exactly does hiking in hot, humid weather seem so much more exhausting?

Question

Assuming one is well hydrated, wears light clothes and hikes in the shade, why exactly does hiking in hot, humid weather seem so much more exhausting than the same hike in cooler weather?

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Background and motivation for posting this, though all I need the answer to the main question above:

There is a hill I frequently walk up on a combination of stone steps and incline, and in the last decade my time has gotten shorter (now about 15 minutes) it feels easier because I've been more active.

However during the summer here the same hike also seems to have gotten more exhausting here, but only during hot and humid days, say above 33 C and 80% humidity.

Answer 1

The effect of temperature on exercise performance has been studied in some detail. This paper nicely reviews a range of hypotheses as well as including experimental data. They seem to conclude that metabolic effects dominate, with the availability of glycogen being a key factor. Effectively your body is less efficient running hotter (and the internal temperature does rise).

High humidity reduces the body's ability to cool by sweating, so acts like higher temperature. This paper has more on humidity specifically, but may not be accessible.

The temperature of the head (specifically the brain) is crucial too, and if you're wearing a hat to keep the sun off, your head will get hotter - so maybe take it off in the shade.

It also applies to running and other activities (my fastest runs have all been in conditions where I'd have got cold if I'd slowed to a walk for more than a couple of minutes). There is a difference though - in some activities, such as cycling and to a lesser extent running, you generate your own cooling headwind. Walking you don't really, though walking into a light breeze is cooler than walking with it at your back.