O Clima do Vale do Rio Doce

Resumo

Climatic conditions in the Rio Doce basin are both varied and moderate. The variety arises as much from the irregularity of the ground as from the geographical position of the basin, which comes under marine and continental influences; furthermore, it is bracketed between the tropics to the north and the temperate zone to the south, and shares some of the characteristics of either.

            The moderate nature of the climate is likewise related to the relief and to the geographical location, the former shielding the area from incursions of air masses so that it is not liable to heavy frontal precipitations and is marked by intermediate pluviometric types such as "humid" and "semi-humid". The average number of rainy days (110 per annum) is another sign of moderation.

            The geographic position of the basin in the transition belt between the tropical and temperate zones of the country explains the moderate temperature in the valley of this important river, revealed in the thermic type, which is mild temperate, and in an attenuation of the characteristics of the tropical and temperate types nearby.

            The distribution of the mean annual temperature is to be seen on the first of the three diagrams that illustrate this paper. It will be noted that the mean temperature ranges from 23 to 18° C. (73 to 64" F.) maxima, with a tendency to increase regularly from west to east. The diagram in question indicates the existence of two thermic types: "temperate" to the west and "tropical" to the east, · with a mild zone in between. The author points out that the warmest time of year is the December-January-February quarter, the temperature rising the highest in February. On the other hand, the June-July-August quarter is the coldest season and the Iowest temperatures are recorded in July.

            The distribution of the rainfall is shown on a diagram of annual isohyets, from which it may be deduced that the Rio Doce valley is an area of sharp contrasts where the yearly total varies from about 27 inches to rather more than 75 inches.

            It will also be seen that the basin can be divided into two parts: one to the west, as far as longitude 42º30', in which the decrease in rainfall (75 to 50 inches) proceeds from west to east, and the other to the east, where this decrease (50 to 27 inches) follows a nort-easterly trend.

            Thus there may be said to be three different zones of rainfall in the Rio Doce valley: semi-humid (below 50 in.), humid (from 50 to 75 in.) and super-humid (above 75 in.).

            The annual distribution of the rainfall is practically identical throughout the valley owing to its being sheltered from the encroachment of polar air masses (continental or marine) and equatorial marine masses (from the North or South Atlantic), except for the lower part of the littoral façade, where "front" rains are recorded.

            In summer, the prevalence of the unstable "equatorial continental" air mass is responsible for the heavy rains in the valley. The relief makes no difference to the pattern of the annual distribution of rainfall, but has an influence on the increase in the pluviometric totals.

            With the exception of the lower valley, the rains fall in the summer (October to March), with some slight variation in the beginning and end of the rainy season, and December nearly always the month of heaviest precipitation. As to the dry season, this generally corresponds to the June-July-August quarter, July being most often the driest month. In the lower valley, however, to the east of the city of Colatina, it rains quite plentifully all the year round and hence this part of the area under survey may be rated "iso-semi-humid".

            With regard to the percentile distribution of the rainfall over the four seasons of the year, lt may be said, generally speaking. that 5% of the total rains fall in winter (June-July-August), 50% in summer (December-January-February), 25% in spring (September-October-November) and 20% in autumn (March-April-May). These proportions are observable throughout the valley except to the east of Aimorés where the rainfall is more evenly distributed and generally breaks down in to 15% in winter, 32% in summer, 28% in spring and 25% in autumn.

            The number of days when it rains, without taking into account the actual rainfall, averages 110 a year, but greater incidence is recorded in the western part of the basin (150 days) and less in the northeastern part (70 days); these figures lead to the conclusion that the frequency of rainy days is relatively low in _comparison with other regions of the country, where it rains 200 to 250 days in the year.

            The heaviest rains occur in the period from October to March in the form of showers, particularly in December and March. It should be noted that the rate of rainfall oscillates .around 4 inches in 24 hours, during the heaviest downpours, which is quite moderate compared with as much as 16 inches recorded in other parts of the country. As a general rule the heaviest rains fall in the south and west of the basin, abating towards the northeast.

            In the valley of the Rio Doce, 5 climatic types are to be distinguished, laid out in roughly parallel belts from east to west: They are: 1) Tropical iso-semi-humid, in the lower valley, from the ocean to the mouth of the Manhuaçu; 2) Tropical semi-humid, in the middle valley, up to the mouth of the Suaçui Grande; 3) Mild temperate semi-humid, in the middle valley, up to the mouth of the Piracicaba; 4) Temperate humid, in the rest of the basin; 5) Temperate super-humid, around Ouro Preto.

            The most extensive is the temperate humid belt.

            As regards the rainy season, two varieties are to be distinguished: Vº', which indicates summer rains, starting early and ending late, and ISO, indicating a fairly even distribution of the rainfall throughout all the months in the year. According to the Kõppen classification, there are only 3 types of climate to be found in the Rio Doce valley as regards temperature, and as regards rainfall merely: 1) Aw, tropical with dry winter, in the east of the basin, low altitude; 2) Cwa, mild with dry winter in the rest of the basin, except along the western and southern edges; and 3) Cws, temperate with dry winter, on the western and southern edges oi the basin. This sort of classification smooths out all diversity, lumping together in the same Aw type as widely varying points as Resplendor with less than 700 mm. (27.5 in.) rainfall a year - the driest in the basin - and Antônio Dias, with 1600 mm. (63 in.) a year, which amounts to quite a rainy climate.

            The Rio Doce valley comprises a reasonable variety of climatic types of no great severity, and, generally speaking, the climate of the valley may be considered quite favorable for human activities.