| Fig. 98—Dense ground cover, typical trees, epiphytes, and parasites of the tropical rain forest at 2,500-3,000 feet between Pongo de Mainique and Rosalina. | Fig. 99—The Urubamba Valley below Santa Ana. On the dry valley floor is a mixed growth of scattered trees, shrubs and grass, with shrubs predominating. Higher up a more luxuriant ravine vegetation appears. On the upper spurs true forest patches occupy the shady slopes. Finally, in the zone of clouds at the top of the picture is a continuous forest. See [17] , for regional applications. |
Combined with these topographic features are certain climatic features of equal precision. Between 7,000 and 13,000 feet is a zone of clouds oftentimes marked out as distinctly as the belt of fog on the Peruvian coast.[26] Rarely does it extend across the valley. Generally it hangs as a white belt on the opposite walls. When the up-valley winds of day begin to blow it drifts up-valley, oftentimes to be dissolved as it strikes the warmer slopes of the upper valley, just as its settling under surface is constantly being dissolved in the warm dry air of the valley floor. Where the precipitation is heaviest there is a belt of woodland—dark, twisted trees, moss-draped, wet—a Druid forest. Below and above the woodland are grassy slopes. At Incahuasi a spur runs out and down until at last it terminates between two deep canyons. No ordinary wells could be successful. The ground water must be a thousand feet down, so a canal, a tiny thing only a few inches wide and deep, has been cut away up to a woodland stream. Thence the water is carried down by a contour-like course out of the woodland into the pasture, and so down to the narrow part of the spur where there is pasture but no springs or streams.
Corn fields surround the few scattered habitations that have been built just above the break or shoulder on the valley wall where the woodland terminates, and there are fine grazing lands. The trails follow the upper slopes whose gentler contours permit a certain liberty of movement. Then the way plunges downward over a staircase trail, over steep boulder-strewn slopes to the arid floor of a tributary where nature has built a graded route. And so to the still more arid floor of the main valley, where the ample and moderate slopes of the alluvial fans with their mountain streams permit plantation agriculture again to come in.
To these three climates, the western border type, the eastern border type, and the inter-Andean type, we have given chief attention because they have the most important human relations. The statistical records of the expedition as shown in the curves and the discussion that accompanies them give attention to those climatic features that are of theoretical rather than practical interest, and are largely concerned with the conventional expression of the facts of weather and climate. They are therefore combined in the following chapter which is devoted chiefly to a technical discussion of the meteorology as distinguished from the climatology of the Peruvian Andes.
CHAPTER X
METEOROLOGICAL RECORDS FROM THE PERUVIAN ANDES
Introduction
The data in this chapter, on the weather and climate of the Peruvian Andes, were gathered under the usual difficulties that accompany the collection of records at camps scarcely ever pitched at the same elevation or with the same exposure two days in succession. Some of them, and I may add, the best, were contributed by volunteer observers at fixed stations. The observations are not confined to the field of the Yale Peruvian Expedition of 1911, but include also observations from Professor Hiram Bingham’s Expeditions of 1912 and 1914-15, together with data from the Yale South American Expedition of 1907. In addition I have used observations supplied by the Morococha Mining Company through J. P. Little. Some hitherto unpublished observations from Cochabamba, Bolivia, gathered by Herr Krüger at considerable expense of money for instruments and of time from a large business, are also included, and he deserves the more credit for his generous gift of these data since they were collected for scientific purposes only and not in connection with enterprises in which they might be of pecuniary value. My only excuse to Herr Krüger for this long delay in publication (they were put into my hands in 1907) is that I have wanted to publish his data in a dignified form and also to use them for comparison with the data of other climatic provinces.