Weather is probably a major factor when you plan your trip to Porto Velho, and keep in mind that cheaper options are usually available when the weather isn't as good. The hottest months are usually September and August with an average temp of 82°F, while the coldest months are June and July with an average of 79°F. The rainiest months in Porto Velho are March, February, January, and December, with each month seeing an average of 16 inches of rainfall.