Humidity: This grand and universal source of vegetable life in high latitudes is infinitely more detrimental to man than even the highest solar heat. Humidity, indeed, is the great modifier of all climates, and constitutes the chief element of their insalubrity. The hygrometrical variations of Brazil have been studied by numerous observers, amongst whom the most accurate as well as the most recent is M. Pissis, and to his conclusions we shall briefly allude, confining ourselves to the climate of the capital, Rio de Janeiro, which, notwithstanding its clear atmosphere, holds in solution just double the quantity of aqueous vapour sustained by the sombre, foggy air of Paris! a fact explained however by the high temperature of the one, as compared with the low temperature of the other, the capacity of air for retaining moisture being in nearly exact proportion to its temperature. M. Pissis arrives at the following results:—
1. From May to October, when the air is serene, the quantity of vapour varies little throughout the day. During the other months, the minimum corresponds with sunrise, and attains its maximum about 4 p.m.; but the variations are trifling.
2. That on rainy days the air is always near its point of saturation, though the amount of vapour dissolved little exceeds that of the preceding clear weather: this is due to the lower temperature of the rainy days.
3. That humidity increases from the month of June to February, when it attains its maximum, which is about double that of June; from this maximum it declines until it reaches its former amount in June and July.
4. That the absorbing power of the air is lowest at sunrise, and attains its maximum about 2 p.m., the hottest period of the day. In like manner as regards the year, it augments in proportion as the sun advances to the southern tropic, and attains its maximum in December and January, and then declines until the cloudy months of June and July.
Rain: The wet season sets in at different epochs along the coast of Brazil, and is subject to great variation. At Rio the rains commonly commence in March, and last till September; at St. Paul, in October and November, and continue till April; whilst at St. Catherine the four seasons are, as in Europe, pretty distinctly defined—July and the following three months wet, cloudy, and boisterous. These latter provinces, placed just beyond the Tropic of Capricorn, enjoy the advantages of a tropical climate without its inconveniences. Rio Grande do Sul is wet and stormy in the winter months, but otherwise healthy. In the provinces north of Rio, including Bahia and Pernambuco, the rains set in commonly about the end of March, and continue until August; and as we follow the coast to the equator, including the provinces of Ceara, Maranham, and Pará, storms are frequent, and the rains commence in December or January; August, September, October, and November being the dryest or summer months. The foregoing may be taken as the rule, but the exceptions are numerous; and the winter of the coast does not extend beyond 100 miles into the interior, which is watered, chiefly, by frequent storms.
Winds: The general winds of tropical regions are eastern; and in Brazil the prevailing currents along the coast, from St. Catherine to Maranham, are E.S.E., and S.S.E., during the southern, and E.N.E. and N.N.E. during the northern monsoon; subject however to much irregularity. The land breeze sets in from 9 to 11 p.m., and lasts till morning, increasing in force and regularity as we approach the equator; and its strength is generally in proportion to that of the sea breeze which precedes it. As in other tropical countries, the sea breeze prevails more in the hot, and the land breeze in the cold season of the year; they favour the appearance of certain maladies and check others, and constitute, after heat and moisture the chief element in the determination of disease—the salubrity of any country depending more, perhaps, on its winds than on its latitude.
Electricity: All tropical regions are distinguished by intensity of electrical phenomena, and Brazil forms no exception to the law. Réaumur maintains, and we believe justly, that a difference of 5° in the thermometer decidedly affects the nervous system; and that all living organisms are powerfully influenced by electrical changes no close observer in equatorial regions can for an instant question. In Brazil, the most intense variations are noticed about the change of the monsoons, and the storms of lightning and thunder originating in the great chain of the Organ Mountains, which burst over Rio, are grand and awful beyond the possibility of description; whilst the profound influence of these changes on individuals is strongly pourtrayed in the moral and physical prostration of some, and the high nervous excitement of others. Saussure has shown that an excess of watery saturation diminishes atmospheric pressure; and the effect of certain conditions of the atmosphere on the human economy in tropical climates cannot for a moment be denied: for example, when the weather is wet and cloudy, the sun obscured, and the air calm; all animal life languishes. The Brazilians distinguish this state of atmosphere by a particular term, ‘mormaço,’ and during its continuance, especially in summer, the mental and bodily powers of man seem alike paralysed, and are only restored to activity when the rain has descended and the breeze resumed its power over the close and stagnant atmosphere. Here electricity plays an important part. In connection with this subject, it is remarkable that Brazil should have hitherto escaped those formidable earthquakes which have so often desolated the fairest regions of South America. Fogs are rare in Brazil, and seen only in the morning, on low and marshy grounds, and in the neighbourhood of rivers and lakes. Hail often falls in Minas, St. Paul, and the south, and even occasionally at Rio. Ice is sometimes met with at Rio Grande in the winter, and even on the Organ Mountains, close to Rio, but never snow. Waterspouts have been, at long intervals, observed on the coast and in the interior; the last of any importance was observed at San Marcos in 1823.
Based on the foregoing and other data, we shall now submit certain general conclusions on the climate of Brazil, and its influence on the human constitution in health and disease; these conclusions must be taken as more especially referring to the seaboard and the large cities on the coast; and the reader should bear in mind that some allowance must also be made for the difference in position and latitude of the northern, the southern, and the central provinces. We would further premise, that these observations are founded on our own personal experience of nearly a quarter of a century, and prior to the advent of the yellow fever which, for the last four or five years, has infested the maritime cities of the empire, and on which we shall presently offer some remarks.
The great characteristic of Brazil, as compared with other countries, is the general equability of its climate, and which constitutes, in fact, the chief element of its salubrity. This unparalleled uniformity of temperature must be chiefly ascribed to the absence of high and mountainous regions, and of all arid and sandy deserts, aided by the genial influence of refreshing showers at all seasons of the year; it is further maintained by the perpetual verdure of the country, and by a cool, powerful, and never-failing monsoon, laden with moisture, and sweeping along the entire line of coast direct from the Southern Atlantic. Thus, even in the height of summer, the diurnal heat is rarely found oppressive to the European, and the nights are almost invariably serene and beautiful, and unattended with much deposition of dew, especially in the northern and central provinces; so that the delightful coolness of tropical moonlight may be enjoyed undisturbed by those visions of fever and malaria which float before the imagination in less favoured lands. If precautions be observed to avoid exposure to direct currents of air, the windows of the sleeping chamber may also be left open with impunity at all seasons of the year; an advantage that can scarcely be over estimated in high latitudes, as disposing to sound and refreshing sleep; which, more perhaps than any other influence enables the European constitution to resist the deleterious effects of climate, just as a succession of hot and sleepless nights invariably predisposes the human system to be impressed by every tropical malady.