After many years of faithful adherence to my personal decision to avoid southern Europe in the summer months, I am heading to Provence and the French Riviera in mid-August. From your August experiences there, in terms of heat and humidity just how miserable is it likely to be? I am choosing hotels with air conditioning of course. But outside of hotels and restaurants, etc., what will it be like; how hot and humid? I don't find historical averages very useful for current times, so I'm asking about your experiences if you have been in those areas in the summer months recently. Is it possible I will be pleasantly surprised? Or will my fears about temps in the 90s most likely be true?

(I'm not looking for advice NOT to go there in August; this is not my decision).

weatherspark.com has a "muggy" index, along with other excellent climate data.

Well, not recent but… I was in Provence for a month in July, 2015… I lost 10 pounds! Upside was I could eat as much fois gras and rich food and wine as I wanted! 😂

My advice would be to not have the AC set too cold. Your body will adjust to heat but not if you keep going from cool to hot and back again. That just confuses it. Sure set it to a cooler temp so you can sleep but even then not too cool, and try not to rely on AC during the day.
Plenty of people visit Provence in August, without AC, and they all survive.
Oh and slow down. Don't try to cram too much into a day. Allow time to sit in the shade and relax a little. Avoid the hottest time of day for activity. Dress appropriately, maybe wear a hat.

Temperatures got up to 40° on certain days last summer. That doesn't mean that it will happen next summer.

A lot depends on what you’re used to and your tolerance for different weather.

If you’re going anyway, you can plan some small things to make sure the weather doesn’t bother you so much. Small things can make a difference, such as has been said about not creating too much difference between inside and outside temperatures; wearing cotton and linen; relaxing during the hottest part of the day; getting up early and going to bed late; having a small misting bottle filled with water; drinking lots; wearing a hat and sunscreen; not thinking about the weather too much, etc, etc. All obvious things of course.

it depends on activity and architecture. Last time I stayed in a castle with staff controlled windows (southern ones shuttered during the day, northern doors left open etc), toured during the early morning and evening, spent days in shady garden on top of a ridge for the breeze, no AC required

Wore a lot of linen, kept covered up

If it's a really hot summer it will be really hot, but not like Palm Springs or Seville, where it is like walking on the sun.

Even then, I would not worry or change my plans. Bring a wide brimmed hat. That way you can carry shade around with you wherever you are.

Coastal is always more tolerable because of sea breezes, so don't sleep on the port of Marseille or the boat ride out to Port Frioul and Chateau d'If. Les Calanques. Higher elevation is also a way to reduce the heat.

Stating the obvious, but the tip that Rick Steves has had for so many summers now: place a filled water bottle into your fridge as you go to sleep.
The following day's activities (daytrip/stroll/simple shopping) will feel easier with some cool water running down the throat.

Joanny...TEN POUNDS! jayzus
I am done. the heat