I first visited SP around the July and August period (from the UK) and found it a little cool, mainly because I had the perception that no matter when you visit it is hot. This is not entirely the case, especially in SP.. SP can get cool but it is not so extreme that you will need more than a light jacket / sweater but I would have found it cold in shorts and t-shirt.
I would say it is probably like London in October time - cool start to the day, warms up and then gets cool again