Okay, I will use Budapest as an example because I live here and its easier for me, but there are lots of places where it isnt as hot as it appears on the thermometer.
Today in Budapest it got to 98F. That’s hot. But how hot? The average temperature in August in Atlanta GA is only 90F, but Budapest at 98F is still 13F cooler.
To understand how the heat treats your body they have developed a Heat Index that recognizes its not just the temperature, but its as much the humidity. https://www.wpc.ncep.noaa.gov/html/heatindex.shtml
90F in Atlanta (the August average) at 75% humidity (the August average) ranks a heat index of 110F
98F here in Budapest at 25% humidity (actual today) ranks a heat index of 97F
For all practical purposes it’s 13F cooler here than it is in cooler Atlanta.
When I was in practice we had a table that gave credit for air movement, but I can’t find it on-line. Suffice it to say the constant 6mph breeze here increases the comfort level more.
When you look up temperatures, also look up the humidity and the heat index to get a true feeling for what you will be getting into.
Then if someone has a question about summer travel, why don’t we first suggest the cooler places in the region that they are interested in. There are beautiful places in Spain that rarely get above 80F in the Summer. Madrid can reach 90F in July but the average humidity there is only 35% which makes it a lot more bearable. Barcelona, not so much. There are cooler places in Greece too. Sure hit the typical tourist hot spots but mix in some cooler weather too?