Well, to me warm is 70 to 80. I find temps above 80 too hot to do anything comfortably without air conditioning - especially sleep. And above 95 in unbearable. I know a lot of people say low humidity helps - but I've never been in Venice when it wasn't humid. So I vote for the AC for anytime in the summer - and yes you will need it in Florence and Rome just as much as in Venice.

(And yes, I've been to the Southwest -never again. 100 and dry is really unbearable - who wants to spend their whole life sweating?)

As a PS to my last post, YES it was and we did sweat the August we were there, as it was in the mid to high 90s in the sun during the hottest time of day. BUT it did not smell bad. Ever. Again, we dressed lightly & enjoyed ourselves immensely. But, would never consider Venice in high summer heat without A/C. With young children, to me it would be madness not to have A/C!