Popular Science Monthly/Volume 67/July 1905/The Climate of the Central American Plateau
| THE CLIMATE OF THE CENTRAL AMERICAN PLATEAU. |
By GUSTAVE MICHAUD, D.Sc,
SPRINGFIELD, MASS.
A FEW weeks ago the Springfield Republican published the following extract from a letter sent by our minister to Costa Rica, Hon. W. L. Merry, to Gen. G. W. Davis, governor of the Panama canal zone:
The writer spent six years, from 1889 to 1895, on the Central American plateau and gathered some meteorological and physiological data which led him to the conviction that our minister's statement is not exaggerated. The climate of that portion of the upland which extends from the Panama isthmus to the Yucatan peninsula, and which includes the highlands of Costa Rica, Nicaragua, Honduras and San Salvador, has features of its own, not frequently found under the tropics and never outside of them.
The uniformity of temperature throughout the year, which characterizes the tropical climate, an altitude of some 1,000 feet above sea level, with its corresponding decrease in the density of air, a lower temperature than could be expected for such an altitude in the torrid zone, are the main features of the climate of the Central American plateau. The last of these is the most important. The value of tropical plateaus as health resorts is a subject which has been much discussed recently. That of the Central American upland will be better understood if some of the conclusions which have been reached within the last ten years, as well as some of the experiments which have led to these conclusions, are previously stated.
The density of air decreases rapidly as one rises on a mountain slope. At an altitude of 18,500 feet, a given volume of air contains but one half the quantity of oxygen which it contains at sea-level. Scarcity of oxygen does not seem to be a desirable condition, yet recent experiments have shown that, within certain limits, that very quality of mountain air induces in the human system changes which are beneficial to the majority of patients. The most important of these can be briefly stated: The number of the blood red corpuscles increases as the air becomes rarer. The main office of the red corpuscles is the absorption of oxygen in the lungs and the carrying of it into the whole body, wherever heat and power are to be generated. There are over two millions red corpuscles 
Noon at the Village Market, Costa Rica. in one drop of the blood of a healthy person. In anæmia and connected states, that number often falls to one half of its value. Life in rarefied air results in the opposite state. After the adaptation period is over, a considerable increase in the number of red corpuscles, coinciding with more frequent and deeper aspirations, causes the paradoxical result, that more oxygen is brought to the organs in the rarefied mountain air than in the denser medium, at sea-level.
The scientist, Viault, was the first to notice that there was an extraordinary number of red corpuscles in the blood of the inhabitants of the high plateaus of Peru. Careful determinations led him to the conclusion that the average increase was from 
Shade and Light Effects at a High Altitude, Carpenteria Mountain. 5,000,000 per cubic millimeter in the blood of a man living at sea-level, to 8,000,000 after a stay of three weeks at an altitude of some eight thousand feet. In their book, La vie sur les hauts plateaux, the Doctors Herrera and Lope, of the city of Mexico, reached a similar conclusion. In Europe, Egger experimented on Monte Rosa (6,201 feet), and found that the red corpuscles increased, on the average, 17 per cent, in two weeks. Karcher, Sutter, Veillon, experimenting on lower altitudes (3,452, 3,232, 2,297 feet), still found a notable increase of the red corpuscles. Wolff and Koeppe noticed it again on the Reyboldsgrun, which is only 2,300 feet high. In 1896, Leuch published in the Korrespondenzblatt f. Schweitzer Aerzte, the results of a most accurate and painstaking study on the changes undergone by the blood of anæmic school children after a stay in the mountains. The anæmia disappeared, while the number of red corpuscles increased. It decreased after return to a lower level, but remained higher than before the sojourn on the mountain. These facts were fully confirmed by the researches of Mercier, von Jaruntowsky and Schroder.
The greater consumption of oxygen induced by the increase in the number of red corpuscles has been measured. According to Schumberg and Zuntz, a man brought to an altitude of 12,467 feet consumes 33 per cent, more oxygen than at sea-level. The increase is not instantaneous. Coindet found that foreigners who had but recently arrived on the Mexican plateau inspired 5.5 liters per minute, while those who had spent a longer period in the same localities took in nearly 6.5 liters.
But while the adaptive changes take place readily and regularly under ordinary conditions in healthy persons, many observations at health resorts of high altitude have shown that in a few cases (old age, certain organic diseases of the heart) they do not take place at all, while in the case of anæmic and neurotic patients they take place only at a moderate altitude and when a cool and bracing atmosphere allows of much outdoor exercise without perspiration. A natural inference to be drawn from those facts is that the climates of all tropical plateaus are not equally conducive to health. While all of them allow life outdoors in any season, and, when not extremely high, are absolutely free from the considerable and sudden changes of temperature which are so injurious to consumptives in the temperate zone, yet the value of such resorts depends mainly on the grade of cooling attending the ascent, especially in the case of general debility, constitutional or even symptomatic. Extremely high altitudes do not agree with the majority of patients; if, at a moderate height (from 3,000 to 5,000 feet), the temperature is low enough to invite exercise, the climate is certainly curative. But if the thermometer reaches daily the eighties, the heart will be unduly quickened during exercise, perspiration will become visible, a tired feeling will appear and the hematose as well as the genesis of blood corpuscles will be interfered with. To suffer from the heat while taking exercise is never invigorating, but, in rarefied air, it is an inconvenience which is the more serious in proportion as the altitude at which it manifests itself is greater. It may be said that, other circumstances being equal, the invigorating value of the climate of a tropical plateau depends on the amount of cold bought at the expense of air rarefaction.
This fact gives the climate of the Central American upland its superiority over that of the broader portion of the plateau which extends from Guatemala to California and which includes the whole Mexican tropical highland. The average yearly temperature of San 
11,500 Feet. Summit of Volcano Irazu.
7,000 Feet. On the Sierra Candelaria.
5,000 Feet. A Street in San Rafael.
3,800 Feet. A Storm on the Volcano Irazu.
2,500 Feet. Rio Brazil.
Sea Level. A Public Garden in Port Limon.
José is 67.5°; that of the sea coast is 78.0°. These figures show an average decrease of temperature of 2.8° for every 1,000 feet of altitude. Observations made in other Central American cities, though less reliable, give even higher figures. On the Mexican tropical plateau, the average decrease of temperature resulting of the ascent is only 2.3° per 1,000 feet. This average is based upon the study of the climate of the cities of Puebla, Mexico, Guadalajare, San Luis Potosi, Pachuca, Zacatecas, Guanajuato, Oaxaca, Queretaro, Aguas Calientes, Leon, Pabellon, San Juan del Rio, Patzecuaro, Tacubaya, the list including all the cities situated on the tropical plateau and for which meteorological records are available. The highest figure (2.7°) is given by the city of Zacatecas, 8,187 feet high; the lowest (1.8°) by the city
of Oaxaca, which, although standing 1,319 feet above San José, has a warmer climate (69.0°) than the latter city.
There are in Central America but two seasons: the rainy season, extending from about the middle of May till the middle of November, and the dry season, covering the rest of the year. In the rainy season, rain falls only for a few hours every day, generally from 2 p. m. till 6 p. m. leaving both mornings and evenings cloudless. Patients suffering from lung troubles find both seasons about equally curative on the highland. Debilitated and neurasthenic invalids recuperate better in the dry than in the rainy term. During the rainy season, the cities situated on the Atlantic slope have a climate somewhat dryer than that of the cities which are located on the Pacific side. The contrary is true during the dry season.
The intensity of light, a result of altitude and latitude combined, is perhaps one of the most important therapeutic elements of the climate of many tropical uplands. Thanks to the researches of Pettenkofer, Voit, and especially to the heliotropic experiments carried on a large scale by Finsen at the Light Institute of Copenhagen, we begin to realize that the chemical rays of sunlight have as much to do with nerve action and metabolism as with sunburn. So far as the latter phenomenon is concerned, the worst sunburn the writer ever saw. . . and felt. . . was got through a colored shirt, in the neighborhood of the city of San José. About 8,000 or 10,000 feet, it is impossible not to notice the extraordinary intensity of solar illumination when the sun is nearing the zenith. At a much lower altitude, in the case of such subjects as 'Noon at the village market,' the photographer soon learns that times of exposure which gave the best results in the United States give overexposed negatives on the Central American plateau.
From one of the scientific periodicals published in the progressive little republic of Costa Rica, El Boletin del Instituto Fisico-Geografico, the writer compiled the following summary of the climatic conditions of the city of San José. The Boletin is published by the government meteorological observatory, an institution founded in 1889 by that remarkable Costa Rican, Don Mauro Fernandez, who was then Minister of Public Instruction. The observatory has a staff of four scientists and has rendered considerable service to the study of the climatology of that quarter of the world. The data represents the average of observations extending over a period of twelve years.
| Dry Season, December to April, Inclusive. |
Rainy Season, May to November, Inclusive. | |||
| Temperature | 2 | a. m. | 62 | 63 |
| " | 6 | a. m. | 60 | 62 |
| " | 10 | a. m. | 72 | 73 |
| " | 2 | p. m. | 77 | 76 |
| " | 6 | p. m. | 68 | 65 |
| " | 10 | p. m. | 64 | 65 |
| —— | —— | |||
| Daily average | 67 | 68 | ||
| Rain, inches | 4 | 71 | ||
| Relative humidity | 75 | 84 | ||