Summer Weather in Spain

Of course, lots of us have been in Spain and Europe in August. Yes, it is, on average, very hot and humid whether it is Spain or Italy or anywhere in the south. We lived in the Chicago area so it is comparable with regard to humidity and then some. It is hotter and more humid because you do not get the effect of the shore breezes in the PM. The bigger issue is that you don't have the same level or quality of AC to duck into or even with the hotels except for the very high end. If I had a choice at that time of year it would be to travel farther north into northern Europe as you suggested. Switzerland is terrific at that time of year.

In Portugal, you should at least get some Atlantic breezes. (The July that I was there, it was an improvement over the heat of Morocco.) But I'd also agree with choosing more northerly destinations, Denmark and more of Scandiniavia among them.

Average August weather in Chicago vs Seville.

I live in Washington DC, which has miserable summer weather, but at home I am not out and about essentially all day long. Just about anywhere I go at home is air conditioned. That will not be the case in Europe, where it's common for some museums, small cafes and shoes to have no a/c. What I've learned to do is pick sumner-appropriate regions in the countries I visit. These have worked for me on recent trips:

Dolomites (Italy)
Basque Country, Galicia and points between (Spain)
Normandy and Brittany (France)
Wales
England

I think traveling north from Paris would be a fine idea. Just be aware of the cost of visiting places like Scandinavia and Switzerland and of making last-minute train ticket purchases in the UK.

We were in Portugal early September of 2016. It was upwards of 104 degrees in Lisbon & Sintra. We were told it was "unusual" to be that warm. As we headed North it started to cool down some. By the time we reached Porto, 2 weeks later it was comfortable.

Two of our last three trips to Spain (2011 and 2014) were to the south; in September and October. Between the two trips we covered the area between Cadiz and Murcia. Temperatures were much more plesant in October. During the September trip southern Spain was in a heat wave and temps were near 100 daily. Doeable, just not as enjoyable when walking around. Ensure you carry water with you. We stopped going to Spain between June and August years ago because of the heat and number of tourists.

I wouldn't describe the summer heat in Spain as humid, but rather hot and dry. I've spent August in Washington, Virginia, South Carolina, Georgia and Florida and they were all hot and humid, the heat in Spain is different. The only time I would consider Spain in the summer is if I intend to spend all my time around a pool or on the beach otherwise you'll find little respite from the heat.

Thanks for all the great info everyone! For the first time in ages, it's 80 degrees here in Chicagoland and I'm already whining, so now my plan is Paris and surrounding area, Brussels, Amsterdam, Edinburgh, (maybe Dublin), and London. With over 4 weeks to travel, I think I can fit that all in and still have time to relax and enjoy where I am. Since I only have University holidays off, Spain and Portugal might have to be a winter break trip. I appreciate your sage advice! Now I'm off to the library to exchange all the Spain and Portugal guidebooks for the Amsterdam and London books!

That should work well. Paris can be hot, though, so be sure you have air-conditioned lodging there.
It's conceivable that you'll wish for it in London, too, but a lot of people would roll the dice in London.

Spain is a perfect destination for winter. I'll be in and out of Chicago in July-August, so thanks for reminding me how unpleasant is can be ☺