"...and of course no-one has got A/C there."

There's your key statement. What you have on this thread is a few dozen individual experiences with their trips. ANYPLACE can have a heat wave. Go to any of the weather sites and looks at historical averages and choose someplace that is usually cool enough for you - which will probably include most of norther n Europe.

The problem with any of the places north of the Alps, where they GENERALLY have mild/cold summers is that they don't have wide spread A/C. One summer I spent a few weeks in Sicily - sure it was hot but the hotels, restaurants and car were all air conditioned. We'd tour around in the hot sun for a couple hours but then get in the AC car or go to a restaurant and cool down, and the nights were no problem with good AC in the rooms. Then we went to Switzerland - where it was cool (and some rain) for a week and THEN it got hot. I was never so uncomfortable as I was that week in Lucerne and Zurich where it was over 90 but no air conditioning.

Well here (NE US) 90 degrees is a normal hot day in summer. 100 - which you often get in Italy - is very hot, and 80 is pleasant. But - everyone has different standards. I hate traveling when it's that hot - I just want to sit in a pool or an AC room. That's why we usually travel in May (Oct for second choice).

But there are plenty of places in europe where 90 degree days are very rare - and most days don't even get to 80 - that are wonderful places to visit.

I just like to see Americans going beyond the usual London, Paris, Rome route in Europe. I would go back to Stockholm in a heartbeat. I found Oslo way less exciting, but I did do Bergen and go through the fiord-train route to Oslo before beginning the home exchange. I did like that. I had a home exchange south of Oslo in Fredrikstad.

No one will know what the weather will be like. You can look at the annual weather statistics and one year it will be hot and the next freezing cold.

Chances are temperatures will be cooler up north.