Sao Paulo weather is usually mild, but can get hot, cold or wet. Summer (November-late February) is a sweaty, humid affair with temperatures in the high 90s F/30s C during the day and sometimes not much less at night. On summer afternoons, dark clouds used to roll in, and the city would be drenched in fierce but short tropical downpours. However, this phenomenon has become unpredictable. But when the rains do come, they tend to be focused, and localized flooding is common.
Either side of summer, the weather is more pleasant. Good months to visit are October and April or even May, when it should still be warm enough to catch some sun, but the storms and oppressive heat are gone.
In late June, July and August, it can get quite chilly, especially at night, when temperatures drop to 30-50 F/0-10 C—this is the result of cold-air systems that move into Brazil from Argentina. However, even in winter, the sun can tan (or burn).