If you’re looking forward to lots of outings with your partner, check out these details about year-round temperatures in Rio Branco: The hottest months are usually September and August with an average temp of 81°F, while the coldest months are July and June with an average of 77°F. The rainiest months in Rio Branco are March, February, January, and December, with each month seeing an average of 13 inches of rainfall.