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 polar fronts.
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.