Precipitation levels vary widely. Most of Brazil has moderate rainfall of between 1,000 and 1,500 mm (39 and 59 in) a year, with most of the rain falling in the summer (between December and April) south of the Equator. The Amazon region is notoriously humid, with rainfall generally more than 2,000 mm (79 in) per year and reaching as high as 3,000 mm (118 in) in parts of the western Amazon and near Belém. It is less widely known that, despite high annual precipitation, the Amazon rain forest has a three- to five-month dry season, the timing of which varies according to location north or south of the equator.
High and relatively regular levels of precipitation in the Amazon contrast sharply with the dryness of the semiarid Northeast, where rainfall is highly erratic and there are severe droughts in cycles averaging seven years. The Northeast is the driest part of the country. The region also constitutes the hottest part of Brazil, where during the dry season between May and November, temperatures of more than 38 °C (100 °F) have been recorded. However, the sertão, a region of semidesert vegetation used primarily for low-density ranching, turns green when there is rain. Most of the Center-West has 1,500 to 2,000 mm (59 to 79 in) of rain per year, with a pronounced dry season in the middle of the year, while the South and most of the East is without a distinct dry season.
Because the South Atlantic basin is generally not a favorable environment for their development, Brazil has only rarely experienced tropical cyclones. The country's coastal population centers are therefore not as burdened with the need to prepare for cyclones, as are cities at similar latitudes in the United States and Asia.
Climate classification for Brazil, according to the Köppen criteria.
Although most of Brazil lies in the tropics, more than 60 percent of the population live in areas which are cooled either by altitude, sea winds or polar fronts. While the coastal cities of Rio de Janeiro, Recife and Salvador can get extremely hot, plateau cities such as São Paulo, Brasília and Belo Horizonte have mild climates, and the southern cities of Porto Alegre and Curitiba have mild winters, but while Curitiba has a warm summer due to the average elevation of 934.6 metres (3,066 ft), Porto Alegre has a hot summer, with an average elevation of only 10 metres (33 ft).
Despite the popular image of the Amazon as a region of blistering heat, temperatures of more than 32 °C (90 °F) are in fact rare. The annual average temperature in the region is 22 to 26 °C (72 to 79 °F), with not much variation between the warmest and the coldest months. The hottest part of Brazil is the northeast, where temperatures of more than 38 °C (100 °F) are frequently recorded during the dry season between May and November. Along the Atlantic coast from Recife to Rio de Janeiro, average temperatures range from 23 to 27 °C (73 to 81 °F). Inland, on higher ground, temperatures are lower, ranging from 18 to 21 °C (64 to 70 °F). South of Rio the seasons are more defined and the range of temperatures significantly wider, with the annual average falling between 17 and 19 °C (63 and 66 °F).
Brazil's most intense rain falls around the mouth of the Amazon near the city of Belém, and also in the upper regions of Amazonia where more than 2,000 millimetres (79 in) of rain fall every year. Most of Brazil has moderate rainfall of between 1,000 and 1,500 millimetres (39 and 59 in) a year, most of it coming between December and April. The driest part of the country is the northeast, where rainfall is erratic and the evaporation rate very high, making it difficult to grow crops.
The highest temperature officially registered in Brazil was 44.7 °C (112.5 °F) in Bom Jesus, Piauí state on 21 November 2005. The lowest temperature officially recorded in Brazil was −14 °C (7 °F) in Caçador, Santa Catarina state, on 11 June 1952. However, the summit of Morro da Igreja, a mountain situated in the municipality of Urubici, also in Santa Catarina, recorded a temperature of −17.8 °C (0.0 °F) on 30 June 1996 unofficially.
The latitudinal position around the Tropic of Capricorn, the very uneven topography and disturbed circulation systems greatly influence the climatology of the Southeast and it is quite diverse in temperature. The annual medium temperature ranges from 20 °C (68 °F) as seen on the border between São Paulo and Paraná to 24 °C (75 °F) in the north of Minas Gerais, while in the elevated areas of the Serra do Espinhaço, Serra da Mantiqueira and Serra do Mar the average medium temperature can be below 18 °C (64 °F) due to the combined effect of the latitude with the frequency of the polar currents.
In the summer, mainly in the month of January, the normal average temperatures range from 30 to 32 °C (86 to 90 °F) in the valleys of the rivers São Francisco and Jequitinhonha, in the Zona da Mata (Forest Zone) of Minas Gerais, in the coastal lowlands and to the west of the state of São Paulo.
In the winter, the normal average temperatures range from 6 to 20 °C (43 to 68 °F) with minimum absolute from −4 to 8 °C (25 to 46 °F), the lowest temperatures being at the highest elevations. Vast areas of Minas Gerais and São Paulo register occurrences of frosts, after the passage of the polarfronts.
As far as the incidence of rain is concerned, there are two areas with heavy precipitation: one following the coast and the Serra do Mar, where the rains are precipitated by the southerly currents; and the other from the west of Minas Gerais to the Municipal district of Rio de Janeiro, where the rains are brought by the Westerly system. The annual precipitation total in these areas is in excess of 1,500 mm (59.1 in). In the Serra da Mantiqueira these indexes surpass 1,750 mm (68.9 in), and at the summit of Itatiaia, 2,340 mm (92.1 in).
In the Serra do Mar, in São Paulo, it rains on the average more than 3,600 mm (141.7 in). Near Paranapiacaba and Itapanhaú maximum rainfall was measured at 4,457.8 mm (175.50 in) in one year. In the valleys of the rivers Jequitinhonha and Doce the smallest annual pluviometric indexes are recorded at around 900 mm (35.4 in).
The maximum pluviometric index of the Southeast area usually occurs in January and the minimum in July, while the dry period is usually concentrated in the winter, lasting six months in the case of the valleys of the rivers Jequitinhonha and São Francisco, to as little as two months in the Serra do Mar and Serra da Mantiqueira.
The climatic characterization of the Northeast area is a little complex, and the four systems of circulation that influence the region are denominated Systems of Disturbed Currents of South, North, East and West. The System of disturbed currents of South is represented by the polarmasses that reach the area in the spring-summer, acts in the coastal areas until the south of Bahia, bringing frontal and back-frontals rains. In the winter the polar masses reach even the coast of Pernambuco, while the hinterlands regions remain under the influence of the tropicalmass.
The system of disturbed currents of North, represented by Convergence Intertropical (CIT), produces rain from the summer to the autumn even in Pernambuco, in the vicinity of the Raso da Catarina. On the other hand, the currents of the East are more frequent in the winter and they usually produce abundant rains in the coastal regions, rarely reaching the scarps of the Plateau of Borborema (800 m or 2,625 ft) and of Chapada Diamantina (1,200 m or 3,937 ft).
Finally, the system of currents of the West, brought by the lines of Tropical Instability (IT), occur from the end of spring to the beginning of autumn, rarely reaching the states of Piauí and Maranhão.
Temperatures are high, with annual averages between 20 and 28 °C (68.0 and 82.4 °F), maxima of around 40 °C (104 °F) having been observed in the south of Maranhão and Piauí. The months of winter, mainly June and July, produce minimum temperatures between 12 and 16 °C (53.6 and 60.8 °F) in the coastal regions, much lower in the plateauregions where temperatures of 1 °C (33.8 °F) have been recorded in Chapada Diamantina after the passage of a polar front.
The pluviosity of the area is complex and is source of concern: its annual totals vary from 2,000 mm (78.7 in) to values even lower than 500 mm (19.7 in), as verified in the Raso da Catarina, between Bahia and Pernambuco, and in the depression of Patos in Paraíba. In a general way, the annual medium precipitation in the northeast area is lower than 1,000 mm (39.4 in) - in the city of Cabaceiras, interior of Paraíba, was observed the smallest annual pluviometric index registered in Brazil, 278 mm (10.9 in)/year. Besides it in the interior of this area the rainy period is usually of just two months in the year, sometimes not coming in some years, causing then the denominated regional droughts.
The South region is located below the Tropic of Capricorn, in a temperate zone. It is influenced by the system of disturbed circulation of the South, which produces the rains, mainly in the summer. It is also influenced by the system of disturbed circulation of the West, that brings rains and storms, sometimes hail, producing winds with bursts of 60 to 90 km/h (37.3 to 55.9 mph). Regarding temperatures: the winter is mild and the summer is hot. The annual medium temperatures range from 14 to 22 °C (57.2 to 71.6 °F), and in places with altitudes above 1,100 m (3,609 ft), drops to approximately 10 °C (50 °F). Some parts of the southern region also have an oceanic climate.
In the summer, mainly in January, in the valleys of the rivers Paranapanema, Paraná and Ibicuí-Jacuí, the medium temperature is in excess of 24 °C (75.2 °F), and the medium temperature of the river Uruguay surpasses 26 °C (78.8 °F). The average maximum temperature stays around 24 to 27 °C (75.2 to 80.6 °F) on the elevated surfaces of the plateau and, in the lowest areas, between 30 and 32 °C (86.0 and 89.6 °F).
In the winter, mainly in July, the medium temperature stays relatively low, oscillating between 10 and 15 °C (50 and 59 °F), except for the valleys of the rivers Paranapanema and Paraná, besides the coast of Paraná and Santa Catarina, where the averages are approximately 15 to 18 °C (59.0 to 64.4 °F). The average maximum temperature is also low, around 20 to 24 °C (68.0 to 75.2 °F), in the big valleys and in the coast, and 16 to 20 °C (60.8 to 68.0 °F) in the plateauregion. The average minimum temperature varies from 6 to 12 °C (42.8 to 53.6 °F) , and the thermometer frequently registers temperatures near 0 °C or below, accompanied by frost and snow, in consequence of the invasion of polarmasses.
The annual medium pluviosity oscillates from 1,250 to 2,000 mm (49.2 to 78.7 in), except along the coast of Paraná and west of Santa Catarina, where the values are in excess of 2,000 mm (78.7 in), and in the north of Paraná and in a small coastal area of Santa Catarina, which have lower recordings down to 1,250 mm (49.2 in). The maximum pluviometric indexes occur in the winter and the minimum in the summer throughout almost the whole area.
The north area of Brazil embraces a great part of the Amazon Basin, representing the largest extension of hot and humidforest on the planet. The region has a low elevation (0 to 200 m or 0 to 656 ft) and is crossed by the Equator. There are four main systems of atmospheric circulation that act in the area, they are: system of winds of Northeast (NE) to East (E) of the Atlantic South and Azores, subtropical anticyclones, generally stable in nature; system of winds of West (W) of the mass equatorial continental (mEc); system of winds of North (N) of the Convergence Intertropical (CIT); and system of winds of South (S) of the Polar anticyclone. These last three systems are responsible for variability of the climate and for the rains in the area. With regard to temperatures, the climate is hot, with annual medium temperatures ranging from
24 to 26 °C (75.2 to 78.8 °F).
Regarding pluviosity, there is not a homogeneity as it occur with the temperature. In the mouth of the river Amazonas, in the coast of Pará and in the western section of the area, the total annual pluviometric index exceeds 3,000 mm (118.1 in) in general. In the direction NO-SE, of Roraima to east of Pará there is less rain, with annual totals in the order of 1,500 to 1,700 mm (59.1 to 66.9 in).
The rainy period of the area occurs in summer & autumn, the exception being Roraima and of the north part of Amazonas, where the maximum pluviometric indexes occurs in winter, due to influence of the climatic conditions of the Northern Hemisphere.
Three systems of circulation occur in the Middle-West region: the system of disturbed currents of the West, represented by unstable events during the summer; system of disturbed currents of the North, represented by Convergence Intertropical (CIT), that produces rains in the summer, autumn and winter in the north of the region; and the system of disturbed currents of the South, represented by the polarfronts, invading the area in the winter with great frequency, producing rains of one to three days duration. In the north and south extremes of the region, the annual medium temperature is 22 °C (71.6 °F) and in the Chapadas it varies from 20 to 22 °C (68.0 to 71.6 °F). In the spring and summer, temperatures are commonly high, the average of the hottest month varying from 24 to 26 °C (75.2 to 78.8 °F). The average of the maximum temperatures of September (hotter month) oscillates between 30 and 36 °C (86.0 and 96.8 °F).
Winter is an interesting season, low temperatures occurring quite frequently. This is caused by the polar invasion, that produces the cold weather which is very common at this time of the year. The medium temperature of the coldestmonth oscillates between 15 and 24 °C (59.0 and 75.2 °F), and the average of the minimum temperatures ranges from 8 to 18 °C (46.4 to 64.4 °F). Minimum temperatures are sometimes negative.
The characterization of the pluviosity of the region is almost exclusively due to the system of atmospheric circulation. The annual medium pluviosity varies from 2,000 to 3,000 mm (78.7 to 118.1 in) in the north of Mato Grosso, to 1,250 mm (49.2 in) in the Pantanal mato-grossense.
In spite of this inequality, the region is well provided with rain. Its seasonality is typically tropical, with maximum in the summer and minimum in the winter. More than 70% of the total rain that is accumulated during the year falls from November to March. The winter is excessively dry, because the rains are very rare.