Most of Brazil experiences only moderate temperature changes throughout the year, though southern states like Rio Grande do Sul have more extreme seasonal changes like those in Europe and the US . In general as you go from north to south, the seasonal changes are more defined.
During the summer, which runs from December to February (school holidays coinciding), Rio and the northeast have temperatures in the high 30s. The rest of the year temperatures are generally in the mid-20s to low 30s. The south has much wider temperature variations, ranging from 15°C in the winter (June through August) to 35°C in the summer.
The Amazon region rarely gets hotter than 27°G but it is humid there. As one might expect, considerable rain falls over tropical Amazonia throughout the year. In some parts of the North, December to March is considered winter, since that's when most of the rain falls.
Owing to generally temperate weather year-round, there is no bad time to visit Brazil . But unless you have your heart set on attending Carnaval, you may want to avoid the summer crowds (and heat), and visit from April to November. Treks into the Amazon are best then - especially during July and August, when the downpours don't last for days and the river is still not at its lowest.