Winter is definitely in Rio in August - The bright side is that being the peak of the summer in other latitudes, everybody has their vacation moods high and bright, and Ipanema is a wonderful place to flaunter them.

as the temperature can keep falling a couple of degrees, getting to the 26°C to 18°C (64° - 78° fahrenheit)the days are getting to be almost 12 hours long - Which means cool beach days, and plenty of private space on the sand. this link gives access to detailed  daily, weekly, and monthly weather forecasts, as well as sunrise and sunset times.

with the possibilities of rain lower than in the previous winter months, on average there are just 7 rainy days in the whole month - humidity falls, making it less likely to hit the 100% level and remains between 50% and 70%.