The summer is very hot and wet; so prepare your umbrella if you are coming between december and february.
São Paulo has a subtropical climate, so, generally, the weather is very hot and wet in the summer (december to february) and moderated cold and very dry during the winter (june to august).
So, if you are visiting the city during the summer it´s a good idea to use soft clothes and carry an umbrella, because it is very likely you will see rain at the end of the day. For the winter there´s no need of very heavy clothes. A single hot jacket is already enough, because the temperature is rarely under 10ºC (50ºF).
Despite all that the weather can change very quickly both in the summer and in the winter. So you can never be sure if tomorrow will be hot or cold, wet or dry.
