Roughly as large as the United States or China, Brazil is by far the world's largest nation straddling the equator. For all its size, however, Brazil has relatively few weather surprises, thanks to its low latitude and the absence of any huge mountain ranges.
Amazonia and Mato Grosso
The northwest third of Brazil is dominated by the vast Amazon basin. The entire basin gets at least 1500mm/60in of rain a year, usually in the form of heavy afternoon thunderstorms and a few rounds of nighttime rain. The Amazon's rains are heaviest on either end – at the headwaters and near the Atlantic coast – and somewhat lighter (this being relative) across western Para state. The sea breeze often triggers storms by afternoon near the coast; these drift westward, dumping late-night downpours on western Para and sometimes reaching Manaus in the wee hours. Across the Guiana Highlands of the far north, the rains tend to be concentrated from April to September; along and just south of the Amazon, they're heaviest from November to May, but a downpour can occur there in any month.
Lows across Amazonia are usually within a few degrees of 23°C/73°F and highs are in the neighbourhood of 31°C/88°F. The humidity is predictably high, although it dips a little during the dry season. The temperature may take a rare tumble between June and September with the arrival of a friagem – a cool front from Argentina. Once or twice in a typical year, a friagem makes it into upper Amazonia, sometimes pushing as far east as Manaus. It can bring two or three days of overcast and temperatures that might dip below 20°C/68°F (or even lower, the further south you are).
The seasons are a bit sharper in the Mato Grosso, the vast lowlands – and swamplands, during the wet season – that extend south from the Amazon and west from the Central Plateau. Here, the summer showers and thunderstorms aren't quite as heavy as on the Amazon, and during the May-to-September dry season, weeks may pass without a drop of rain. Even then, a friagem might bring a day of wintertime drizzle, and after it clears, temperatures can plummet. Even at latitude 15°S, it's been known to dip near freezing. As spring comes on, the Mato Grosso starts to roast. By October, most days top 30°C/86°F, and a few spots can hit 40°C/104°F during late spring and summer.
Seasonal shifts are gentle along the stunning shoreline where southeast Brazil meets the Atlantic. From Vitoria to Porto Alegre, the summer months (including the time of Rio's Carnaval) are made for beachgoing. It's hot and humid, with a thunderstorm about every third or fourth day at Rio de Janeiro and Sao Paolo and a bit less often further south, where it's somewhat less muggy. The southernmost coast, around Porto Alegre, usually dips below 20°C/68°F even in mid-summer. Winter brings a succession of cool fronts that wrap around the coast, tempering the tropical heat. Their impact is strongest toward the south. Mid-winter in Porto Alegre is similar to that in Los Angeles, with a few prolonged bouts of rain and coolish temperatures, while the coast from Rio northward typically gets little more than clouds with each front. Even in July, most afternoons in Rio top 25°C/77°F.
Contrary to the usual tropical pattern, the easternmost tip of Brazil – including the coast from Salvador to Recife and Cabo de Sao Roque – gets most of its rain in the autumn and early winter, from March into August. There's still a good deal of sunshine during those wet months. The showers are least frequent and far lighter from October to December. Sultriness reigns throughout the year.
Brazil's Central Plateau offers a respite from the intense coastal heat and humidity. The altitudes are modest, but they make a difference. In Brasilia (just over 1000m/3300ft), afternoons typically stay close to 25°C/79°F in both winter and summer. Most evenings dip below 16°C/60°F from May to August. In winter, the plateau is sunnier, drier and cooler than the coast. The western plateau gets frequent afternoon thunderstorms during the summer, while the rains are lighter but a bit more sustained further east. The southernmost highlands, fronting the Atlantic, are the only ones that get substantial rain in winter; at the highest points, you might see a snowflake.
West of Fortaleza, the coastal downpours arrive in summer and early autumn, typically from about January to May, with light showers the rule otherwise. Both the wet and the dry seasons get wetter as you move toward Belem. Moving inland, you'll find one of Brazil's most distinctive climatic regions, the Nordeste. From northern Bahia to the Rio Grande do Norte, rainfall is puzzlingly sparse – less than 1000mm/39in per year in general, and below 500mm/19in from Petrolina northeast toward the coast. Moreover, the Nordeste's rainfall is exceedingly erratic – it's common for a year to produce far more or far less than average. The worst droughts tend to occur during El Nino years.