Travels on the Amazon

by Alfred Russel Wallace

CLIMATE.

The climate of the Amazon valley seems remarkable for uniformity of temperature, and for a regular supply of moisture. There are, in most parts of it, six months' wet, and six months' dry season, neither of which are so severe as in some other tropical countries. From June to December is the dry, and from January to May the wet season. In the dry season there are a few occasional rains, especially about All Saints' day, in November; and during the wet season there are intervals of fine weather, and often bright mornings, and many days of gentle misty rain.

This is the general character of the climate over the whole of the main stream of the Amazon and its immediate neighbourhood. There are, however, remarkable deviations from this general routine, in particular localities. Pará itself is one of these exceptional places. Here the seasons are so modified, as to render the climate one of the most agreeable in the world. During the whole of the dry season, scarcely ever more than three days or a week passes without a slight thunderstorm and heavy shower, which comes on about four in the afternoon and by six has cleared off again, leaving the atmosphere delightfully pure and cool and all vegetable and animal life refreshed and invigorated. Had I only judged of the climate of Pará from my first residence of a year, I might be thought to have been impressed by the novelty of the tropical climate; but on my return from a three years' sojourn on the Upper Amazon and Rio Negro, I was equally struck with the wonderful freshness and brilliancy of the atmosphere, and the balmy mildness of the evenings, which are certainly not equalled in any other part I have visited.

The wet season has not so many stormy and cloudy days as in other parts. Sunshine and rain alternate, and the days are comparatively bright and cheerful, even when rainy. Generally, the variation of the thermometer in any one day does not exceed 15°; 75° being the lowest, and 90° the highest. The greatest variation in one day is not, I think, ever more than 20°; and in four years, the lowest and highest temperatures were 70° and 95°, giving only an extreme variation of 25°. A more equable climate probably does not exist on the earth. (See Diagram, [Plate IV.])

On the Guiana side of the Amazon, in the islands of Mexiana and Marajo, the seasons are more strongly marked than even higher up the river. In the dry season, for about three months, no rain ever falls; and in the wet it is almost continual.

But it is in the country about the falls of the Rio Negro that the most curious modification of the seasons occur. Here the regular tropical dry season has almost disappeared, and a constant alternation of showers and sunshine occurs, almost all the year round. In the months of June, July, August, and September, when the Amazonian summer is in all its glory, we have here only a little finer weather about June, and then rain again as much as ever; till, in January or February, when the wet season in the Amazon commences, there is generally here a month or two of fine warm weather. It is then that the river, which has been very slowly falling since July, empties rapidly, and in March is generally at its lowest ebb. In the beginning of April it suddenly begins to rise, and by the end of May has risen twenty feet, and then continues slowly rising till July, when it reaches its highest point, and begins to fall with the Amazon. The district of the greatest quantity of rain, or rather of the greatest number of rainy days, seems to be very limited, extending only from a little below the falls of São Gabriel to Marabitanas at the confines of Brazil, where the Pirapocó and Cocoí mountains, and the Serra of Tunuhy, seem to form a separation from the Venezuela district, where there is a more regular summer in the months of December, January, and February.

The water of the Rio Negro in the month of September did not vary in temperature more than two degrees. I unfortunately lost my thermometers, or had intended making a regular series of observations on the waters of the higher parts of the rivers I ascended.

The extreme variation of the barometer at Pará for three years was only three-tenths of an inch (see diagram, [Plate V.]). The mean height, with all the necessary corrections, would seem to be almost exactly thirty inches; I have, however, already given my reasons for believing that there is a considerable difference in the pressure of the atmosphere in the interior of the country. In the month of May some very cold days are said to occur annually on the Upper Amazon and Rio Negro; but I never myself experienced anything of the kind worthy of notice. Many intelligent persons have assured me that the cold is sometimes so severe that the inhabitants suffer much, and, what is much more extraordinary, the fishes in the rivers die of it. Allowing this to be the fact, I am quite unable to account for it, as it is difficult to conceive that a diminution of temperature of five or ten degrees, which is as much as ever takes place, can produce any effect upon them.

I have an authentic account of hail having once fallen on the Upper Amazon, a remarkable occurrence at a place only three degrees south of the equator, and about two hundred feet above the level of the sea. The children were out at play, and brought it to their parents, astonished at a substance falling from the clouds quite new to them, and which was so remarkably cold. The person who told me was a Portuguese, and his information can be perfectly relied on.