It has been found[55] that the changes run in cycles of from thirty to thirty-five years in length and that the northern and southern hemispheres appear to be in opposite phase. For example, since 1885 the snowline in the southern hemisphere has been decreasing in elevation in nine out of twelve cases by the average amount of nine hundred feet. With but a single exception, the snowline in the northern hemisphere has been rising since 1890 with an average increase of five hundred feet in sixteen cases. To be sure, we must recognize that the observations upon which these conclusions rest have unequal value, due both to personal factors and to differences in instrumental methods, but that in spite of these tendencies toward inequality they should agree in establishing a general rise of the snowline in the northern hemisphere and an opposite effect in the southern is of the highest significance.
It must also be realized that snowline observations are altogether too meager and scattered in view of the abundant opportunities for making them, that they should be standardized, and that they must extend over a much longer period before they attain their full value in problems in climatic variations. Once the possible significance of snowline changes is appreciated the number and accuracy of observations on the elevation and local climatic relations of the snowline should rapidly increase.
In 1907 I made a number of observations on the height of the snowline in the Bolivian and Chilean Andes between latitudes 17° and 20° south, and in 1911 extended the work northward into the Peruvian Andes along the seventy-third meridian. It is proposed here to assemble these observations and, upon comparison with published data, to make a few interpretations.
From Central Lagunas, Chile, I went northeastward via Pica and the Huasco Basin to Llica, Bolivia, crossing the Sillilica Pass in May, 1907, at 15,750 feet (4,800 m.). Perpetual snow lay at an estimated height of 2,000-2,500 feet above the pass or 18,000 feet (5,490 m.) above the sea. Two weeks later the Huasco Basin, 14,050 feet (4,280 m.), was covered a half-foot deep with snow and a continuous snow mantle extended down to 13,000 feet. Light snows are reported from 12,000 feet, but they remain a few hours only and are restricted to the height of exceptionally severe winter seasons (June and early July). Three or four distant snow-capped peaks were observed and estimates made of the elevation of the snowline between the Cordillera Sillilica and Llica on the eastern border of the Maritime Cordillera. All observations agreed in giving an elevation much in excess of 17,000 feet. In general the values run from 18,000 to 19,000 feet (5,490 to 5,790 m.). Though the bases of these figures are estimates, it should be noted that a large part of the trail lies between 14,000 and 16,000 feet, passing mountains snow-free at least 2,000 to 3,000 feet higher, and that for general comparisons they have a distinct value.
In the Eastern Cordillera of Bolivia, snow was observed on the summit of the Tunari group of peaks northwest of Cochabamba. Steinmann, who visited the region in 1904, but did not reach the summit of the Tunari group of peaks, concludes that the limit of perpetual snow should be placed above the highest point, 17,300 (5,270 m.); but in July and August, 1907, I saw a rather extensive snow cover over at least the upper 1,000 feet, and what appeared to be a very small glacier. Certain it is that the Cochabamba Indians bring clear blue ice from the Tunari to the principal hotels, just as ice is brought to Cliza from the peaks above Arani. On these grounds I am inclined to place the snowline at 17,000 feet (5,180 m.) near the eastern border of the Eastern Cordillera, latitude 17° S. At 13,000 feet, in July, 1907, snow occurred in patches only on the pass called Abre de Malaga, northeast of Colomi, 13,000 feet, and fell thickly while we were descending the northern slopes toward Corral, so that in the early morning it extended to the cold timber line at 10,000 feet. In a few hours, however, it had vanished from all but the higher and the shadier situations.
In the Vilcanota knot above the divide between the Titicaca and Vilcanota hydrographic systems, the elevation of the snowline was 16,300+ feet (4,970 m.) in September, 1907. On the Cordillera Real of Bolivia it is 17,000 to 17,500 feet on the northeast, but falls to 16,000 feet on the southwest above La Paz. In the first week of July, 1911, snow fell on the streets of Cuzco (11,000 feet) and remained for over an hour. The heights north of San Geronimo (16,000 feet) miss the limit of perpetual snow and are snow-covered only a few months each year.
In taking observations on the snowline along the seventy-third meridian I was fortunate enough to have a topographer the heights of whose stations enabled me to correct the readings of my aneroid barometer whenever these were taken off the line of traverse. Furthermore, the greater height of the passes—15,000 to 17,600 feet—brought me more frequently above the snowline than had been the case in Bolivia and Chile. More detailed observations were made, therefore, not only upon the elevation of the snowline from range to range, but also upon the degree of canting of the snowline on a given range. Studies were also made on the effect of the outline of the valleys upon the extent of the glaciers, the influence on the position of the snowline of mass elevation, precipitation, and cloudiness.
Snow first appears at 14,500 feet (4,320 m.) on the eastern flanks of the Cordillera Vilcapampa, in 13° south latitude. East of this group of ridges and peaks as far as the extreme eastern border of the mountain belt, fifty miles distant, the elevations decrease rapidly to 10,000 feet and lower, with snow remaining on exceptionally high peaks from a few hours to a few months. In the winter season snow falls now and then as low as 11,500 feet, as in the valley below Vilcabamba pueblo in early September, 1911, though it vanishes like mist with the appearance of the sun or the warm up-valley winds from the forest. Storms gather daily about the mountain summits and replenish the perpetual snow above 15,000 feet. In the first pass above Puquiura we encountered heavy snow banks on the northeastern side a hundred feet below the pass (14,500 feet), but on the southwestern or leeward side it is five hundred feet lower. This distribution is explained by the lesser insolation on the southwestern side, the immediate drifting of the clouds from the windward to the leeward slopes, and to the mutual intensification of cause and effect by topographic changes such as the extension of collecting basins and the steeping of the slopes overlooking them with a corresponding increase in the duration of shade.
It is well known that with increase of elevation and therefore of the rarity of the air there is less absorption of the sun’s radiant energy, and a corresponding increase in the degree of insolation. It follows, therefore, that at high altitudes the contrasts between sun and shade temperatures will increase. Frankland[56] has shown that the increase may run as high as 500 per cent between 100 to 10,000 feet above the sea. I have noted a fall of temperature of 15° F. in six minutes, due to the obscuring of the sun by cloud at an elevation of 16,000 feet above Huichihua in the Central Ranges of Peru. Since the sun shines approximately half the time in the snow-covered portions of the mountains and since the tropical Andes are of necessity snow-covered only at lofty elevations, this contrast between shade and sun temperatures is by far the most powerful factor influencing differences in elevation of the snowline in Peru.
To the drifting of the fallen snow is commonly ascribed a large portion of this contrast. I have yet to see any evidence of its action near the snowline, though I have often observed it, especially under a high wind in the early morning hours at considerable elevations above the snowline, as at the summits of lofty peaks. It appears that the lower ranges bearing but a limited amount of snow are not subject to drifting because of the wetness of the snow, and the fact that it is compacted by occasional rains and hail storms. Only the drier snow at higher elevations and under stronger winds can be effectively dislodged.