O clima do vale do Itajaí

Resumo

The Itajaí valley has its situation characterized by the existence of high mountain ranges in the west and south, and of plains and small heights In the east, near the ocean.

            These important geographic factors make themselves strongly felt in the thermic and pluviometric characteristics of the Itajaí valley. As to the former, its effect is - generally speaking - that of softening up, be it through a lowering of the temperatures in the coastal region, be it through its elevated position in the lee of the mountain ranges which intercept the cold and intense southwesterly "pampeiros" gales. As to the latter, the result is the increased rainfall in a southwest -northeast direction, from 1,100 mm to 1,600 mm, consequently creating two contrasting types in the valley: the semi-humid, in the south and extreme west and, the humid type in the rest of the - by the way vaster - region.

            The rainfall in the whole valley is reasonable distributed throughout the year; so that there is - so to say - no dry season: There are, however, clearly defined rainy seasons: a principal one in the Summer (December to March), with a monthly average of 40 mm; and another secondary one in the Spring (September to October), with monthly 120 mm. The least rainy period of the years is the Autumn-Winter season (April to August) with an 85 mm monthly average.

            The existence of enough rain throughout all seasons of the year may be explained by the superposition of three pluviometric systems which occur in the region.

            1° ) The tropical, with its maximum in the Summer coming discontinuities tropics;

            2° ) The antarctic coming across the sea, with its maximum in the Autumn;

            3°) The Antarctic still, this time sweeping across the continent provoking rain in the Winter and, mainly in the Spring.

            Although the blending of these systems last, round the year the rains are heavier in the Summer when the air is less stable, and scarcer in the Winter when the air masses are more stable.

            The floods that occur sometimes in catastrophic, proportions, on the Itajaí river do not simply depend on the abundance rainfalls in the respective season of the year, but on the correlation "time/area/intensity" of individual disturbances, besides certain condition caused by the junction of the rainfalls on the several tributaries.

            Therefore, it is clear, that great floods may happen not only in the principal rainy season Summer - but also - and even with greater intensity, in the Spring and Autumn, and sometimes in the Winter.

The examination of some floods, especially the one in October 1954, shows that the month it happened there was a pluviometric surplus of 66% over the normal rainfall and that the greatest excesses are localized in two regions of the valley: one in the North-East on the Benedito and Testo tributaries, and the other in the South, along the Itajaí do Sul (Southern Itajaí) tributary; on the other hand, the Itajaí-Oeste (Western Itajaí), and the Itajaí-Norte (Northern Itajaí), as well as the Alto-Itajal-Açu (Upper Itajaí-Açu), in spite of great size of their basins, exercise a much lesser influence on the formation of the floods.