I have been in Rome in the summer. It was hot as hell. I have been there in the Winter, it cool and pleasant. It will be cool and pleasant again next fall. I have been in Madrid, Spain in the summer. It was hot. One winter I froze my ass off in Madrid, Spain as it experienced the coldest weather in the last 50 years. Global warming was not mentioned.
It is called weather.
I was in Venice and Rome in December- best time to go. Not many people not terribly cold (but colder in Venice than in Rome). Only downside was return flight to Dulles was cancelled due to winter storm in mid Atlantic states. Had to scramble to shift reservations to three days later. PITA.