In a country the size of Brazil, it is very difficult to make general comments on the climate. At the same moment in time, a temperature of –2° C can be registered in the mountains in the south of the country while the Northeast registers 43° C.
In general, the seasons in the south of the country are well defined – the further to the south, the bigger the differences between seasons. It is cold in winter, hot during the summer. In the north, there are just two basic seasons: winter and summer. What differenciates one from the other is the rain – it is always hot. It doesn’t rain in winter; it doesn’t stop raining during the summer. Or the other way around because, to complicate things even more, in some places winter, and not summer, is the rainy season.
The Weather Channel’s Portuguese web site is an excellent starting point, as it shows historical averages for a sizeable number of destinations. Click, for example, on Rio de Janeiro, and then on Médias e Registro to see monthly average values for rain and temperatures.
The weather is an important factor when it comes to decide what to visit. For instance, it is not worth visiting the Lençóis Maranhenses during the summer, when it doesn’t stop raining and the lagoons are nearly empty. Or it is not worth going to Fernando de Noronha in December, when 2 and 3 meter-high waves make snorkelling impossible near the beach.