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

If it were really miserable, it would not be so overcrowded.

You mentioned that your hotel will have AC. But what about a pool?
FWIW, when finalizing your hotel choices, some heat management options include:
Book a room that gets afternoon shade and has hard surface floors (wood or tile) instead of carpet, esp wall-to-wall carpet. A room that is in the shade all afternoon and evening will feel markedly cooler than one that gets the full brunt of afternoon/early evening sun.

Instead of AC, I would get a room that's in the shade in a hotel with a pool big enough to swim in. And that shade would ideally come from trees and other greenery.

Keep your hotel with AC. Especially in the countryside, you need it not for the heat but for protection against mosquitoes (or noise). I've had a place in the Cote d'Azur for over nine years (and lived elsewhere in Europe) and have stayed throughout Provence.
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The chose you get to the sea, the more humid it will get. But the peak temperature will typically be less, due to the sea and geography. As you get inland in western Provence, the temperatures can exceed 38C/100F (Some of the hottest temperatures ever recorded in France have been north of Avignon). But typically the nights will cool off by quite a bit. Typically in mid August the weather begins to break and the nighttime cooling increases. The last two summers have been very hot, with hot weather lasting through the night, and the current forecast has the potential to be very hot (due to sea warming being recorded).