And put another way, there are people who live and cycle in 100-120F daily who probably have a little more practical experience on how to dress for heat than even TdF riders.

Brian's post is on the money re heat transfer theory. However, in terms of application, it seems to me that there are a couple of gaps between theory and practice.

1/ The issues with high humidity have obviously been pointed out - but it extremely hot/dry heat, the sweat often doesnt stay on your body long enough. In such case, does a wicking base layer provide a benefit? There is probably a sweet spot of ranges when it works, it seems.

2/ If you are wearing a tight-fitting base layer to wick heat, do you always benefit from an outer layer? A loose-fitting outer layer does reduce the heat transfer to the body, but cyclists dont exactly wear dish-dashas, so that gets a bit academic.

I dont know the answer to this, but I am pretty sure that I'll trust practical experience to people extrapolating basic theory into a more complex situation.

I do wear a base layer when I go hiking in the mountains, with the temperatures in the 90s. It is effective, especially when the sun is hot but the air is relatively cool and the amount of sweating is moderate. I sure as hell do NOT wear a base layer when cycling when the temperature exceeds 100-110F and the air itself seems to scorch your lungs.

Works for me.

V.