Paris has a broadly similar climate to London; it is worth visiting any season of the year, and in particular don't be put off going there in summer. Like London, sometimes it's hot and humid, but the other year we spent the whole of August there, and it was cool and quite wet. Incidentally, another myth is that Paris closes down in August - this may have been true years ago, and some top-class restaurants may still shut their doors, but generally everything is open and there are many specioal summer attractions.