CHAPTER IX

CLIMATOLOGY OF THE PERUVIAN ANDES

CLIMATIC BELTS

The noble proportions of the Peruvian Andes and their position in tropical latitudes have given them climatic conditions of great diversity. Moreover, their great breadth and continuously lofty summits have distributed the various climatic types over spaces sufficiently ample to affect large and important groups of people. When we add to this the fact that the topographic types developed on a large scale are distributed at varying elevations, and that upon them depend to a large degree the chief characteristics of the soil, another great factor in human distribution, we are prepared to see that the Peruvian Andes afford some striking illustrations of combined climatic and topographic control over man.

The topographic features in their relations to the people have been discussed in preceding chapters. We shall now examine the corresponding effects of climate. It goes without saying that the topographic and climatic controls cannot and need not be kept rigidly apart. Yet it seems desirable, for all their natural interdependence, to give them separate treatment, since the physical laws upon which their explanations depend are of course entirely distinct. Further, there is an independent group of human responses to detailed climatic features that have little or no connection with either topography or soil.

The chief climatic belts of Peru run roughly from north to south in the direction of the main features of the topography. Between 13° and 18° S., however, the Andes run from northwest to southeast, and in short stretches nearly west-east, with the result that the climatic belts likewise trend westward, a condition well illustrated on the seventy-third meridian. Here are developed important climatic features not found elsewhere in Peru. The trade winds are greatly modified in direction and effects; the northward-trending valleys, so deep as to be secluded from the trades, have floors that are nearly if not quite arid; a restricted coastal region enjoys a heavier rainfall; and the snowline is much more strongly canted from west to east than anywhere else in the long belt of mountains from Patagonia to Venezuela. These exceptional features depend, however, upon precisely the same physical laws as the normal climatic features of the Peruvian Andes. They can, therefore, be more easily understood after attention has been given to the larger aspects of the climatic problem of which they form a part.

The critical relations of trade winds, lofty mountains, and ocean currents that give distinction to Peruvian climate are shown in Figs. 71 to 73. From them and Fig. 74 it is clear that the two sides of the Peruvian mountains are in sharp contrast climatically. The eastern slopes have almost daily rains, even in the dry season, and are clothed with forest. The western leeward slopes are so dry that at 8,000 feet even the most drought-resisting grasses stop--only low shrubs live below this level, and over large areas there is no vegetation whatever. An exception is the Coast Range, not shown on these small maps, but exhibited in the succeeding diagram. These have moderate rains on their seaward (westerly) slopes during some years and grass and shrubby vegetation grow between the arid coastal terraces below them and the parched desert above. The greatest variety of climate is enjoyed by the mountain zone. Its deeper valleys and basins descend to tropical levels; its higher ranges and peaks are snow-covered. Between are the climates of half the world compressed, it may be, between 6,000 and 15,000 feet of elevation and with extremes only a day’s journey apart.

In the explanation of these contrasts we have to deal with relatively simple facts and principles; but the reader who is interested chiefly in the human aspects of the region should turn to p. 138 where the effects of the climate on man are set forth. The ascending trades on the eastern slopes pass successively into atmospheric levels of diminishing pressure; hence they expand, deriving the required energy for expansion from the heat of the air itself. The air thereby cooled has a lower capacity for the retention of water vapor, a function of its temperature; the colder the air the less water vapor it can take up. As long as the actual amount of water vapor in the air is less than that which the air can hold, no rain falls. But the cooling process tends constantly to bring the warm, moist, ascending air currents to the limit of their capacity for water vapor by diminishing the temperature. Eventually the air is saturated and if the capacity diminishes still further through diminishing temperature some of the water vapor must be condensed from a gaseous to a liquid form and be dropped as rain.

The air currents that rise thousands of feet per day on the eastern slopes of the Andes pass again and again through this practically continuous process and the eastern aspect of the mountains is kept rain-soaked the whole year round. For the trades here have only the rarest reversals. Generally they blow from the east day after day and repeat a fixed or average type of weather peculiar to that part of the tropics under their steady domination. During the southern summer, when the day-time temperature contrasts between mountains and plains are strongest, the force of the trade wind is greatly increased and likewise the rapidity of the rain-making processes. Hence there is a distinct seasonal difference in the rainfall--what we call, for want of a better name, a “wet” and a “dry” season.

On the western or seaward slopes of the Peruvian Andes the trade winds descend, and the process of rain-making is reversed to one of rain-taking. The descending air currents are compressed as they reach lower levels where there are progressively higher atmospheric pressures. The energy expended in the process is expressed in the air as heat, whence the descending air gains steadily in temperature and capacity for water vapor, and therefore is a drying wind. Thus the leeward, western slopes of the mountains receive little rain and the lowlands on that side are desert.

THE CLIMATE OF THE COAST

A series of narrow but pronounced climatic zones coincide with the topographic subdivisions of the western slope of the country between the crest of the Maritime Cordillera and the Pacific Ocean. This belted arrangement is diagrammatically shown in Fig. 75. From the zone of lofty mountains with a well-marked summer rainy season descent is made by lower slopes with successively less and less precipitation to the desert strip, where rain is only known at irregular intervals of many years’ duration. Beyond lies the seaward slope of the Coast Range, more or less constantly enveloped in fog and receiving actual rain every few years, and below it is the very narrow band of dry coastal terraces.

The basic cause of the general aridity of the region has already been noted; the peculiar circumstances giving origin to the variety in detail can be briefly stated. They depend upon the meteorologic and hydrographic features of the adjacent portion of the South Pacific Ocean and upon the local topography.

The lofty Andes interrupt the broad sweep of the southeast trades passing over the continent from the Atlantic; and the wind circulation of the Peruvian Coast is governed to a great degree by the high pressure area of the South Pacific. The prevailing winds blow from the south and the southeast, roughly paralleling the coast or, as onshore winds, making a small angle with it. When the Pacific high pressure area is best developed (during the southern winter), the southerly direction of the winds is emphasized, a condition clearly shown on the Pilot Charts of the South Pacific Ocean, issued by the U.S. Hydrographic Office.

The hydrographic feature of greatest importance is the Humboldt Current. To its cold waters is largely due the remarkably low temperatures of the coast.[21] In the latitude of Lima its mean surface temperature is about 10° below normal. Lima itself has a mean annual temperature 4.6° F. below the theoretical value for that latitude, (12° S.). An accompanying curve shows the low temperature of Callao during the winter months. From mid-June to mid-September the mean was 61° F., and the annual mean is only 65.6° F. (18° C.). The reduction in temperature is accompanied by a reduction in the vapor capacity of the super-incumbent air, an effect of which much has been made in explanation of the west-coast desert. That it is a contributing though not exclusive factor is demonstrated in Fig. 77. Curve _A_ represents the hypothetical change of temperature on a mountainous coast with temporary afternoon onshore winds from a _warm_ sea. Curve _B_ represents the change of temperature if the sea be cold (actual case of Peru). The more rapid rise of curve _B_ to the right of X-X′, the line of transition, and its higher elevation above its former saturation level, as contrasted with _A_, indicates greater dryness (lower relative humidity). There has been precipitation in case _A_, but at a higher temperature, hence more water vapor remains in the air after precipitation has ceased. Curve _B_ ultimately rises nearly to the level of _A_, for with less water vapor in the air of case _B_ the temperature rises more rapidly (a general law). Moreover, the higher the temperature the greater the radiation. To summarize, curve _A_ rises more slowly than curve _B_, (1) because of the greater amount of water vapor it contains, which must have its temperature raised with that of the air, and thus absorbs energy which would otherwise go to increase the temperature of the air, and (2) because its loss of heat by radiation is more rapid on account of its higher temperature. We conclude from these principles and deductions that under the given conditions a cold current intensifies, but does not cause the aridity of the west-coast desert.

Curves _a_ and _b_ represent the rise of temperature in two contrasted cases of warm and cold sea with the coastal mountains eliminated, so as to simplify the principle applied to _A_ and _B_. The steeper gradient of _b_ also represents the fact that the lower the initial temperature the dryer will the air become in passing over the warm land. For these two curves the transition line X-X’ coincides with the crest of the Coast Range. It will also be seen that curve _a_ is never so far from the saturation level as curve _b_. Hence, unusual atmospheric disturbances would result in heavier and more frequent showers.

Turning now to local factors we find on the west coast a regional topography that favors a diurnal periodicity of air movement. The strong slopes of the Cordillera and the Coast Range create up-slope or eastward air gradients by day and opposite gradients by night. To this circumstance, in combination with the low temperature of the ocean water and the direction of the prevailing winds, is due the remarkable development of the sea-breeze, without exception the most important meteorological feature of the Peruvian Coast. Several graphic representations are appended to show the dominance of the sea-breeze (see wind roses for Callao, Mollendo, Arica, and Iquique), but interest in the phenomenon is far from being confined to the theoretical. Everywhere along the coast the _virazon_, as the sea-breeze is called in contradistinction to the _terral_ or land-breeze, enters deeply into the affairs of human life. According to its strength it aids or hinders shipping; sailing boats may enter port on it or it may be so violent, as, for example, it commonly is at Pisco, that cargo cannot be loaded or unloaded during the afternoon. On the nitrate pampa of northern Chile (20° to 25° S.) it not infrequently breaks with a roar that heralds its coming an hour in advance. In the Majes Valley (12° S.) it blows gustily for a half-hour and about noon (often by eleven o’clock) it settles down to an uncomfortable gale. For an hour or two before the sea-breeze begins the air is hot and stifling, and dust clouds hover about the traveler. The maximum temperature is attained at this time and not around 2.00 P. M. as is normally the case. Yet so boisterous is the noon wind that the laborers time their siesta by it, and not by the high temperatures of earlier hours. In the afternoon it settles down to a steady, comfortable, and dustless wind, and by nightfall the air is once more calm.

Of highest importance are the effects of the sea-breeze on precipitation. The bold heights of the Coast Range force the nearly or quite saturated air of the sea-wind to rise abruptly several thousand feet, and the adiabatic cooling creates fog, cloud, and even rain on the seaward slope of the mountains. The actual form and amount of precipitation both here and in the interior region vary greatly, according to local conditions and to season and also from year to year. The coast changes height and contour from place to place. At Arica the low coastal chain of northern Chile terminates at the Morro de Arica. Thence northward is a stretch of open coast, with almost no rainfall and little fog. But in the stretch of coast between Mollendo and the Majes Valley a coastal range again becomes prominent. Fog enshrouds the hills almost daily and practically every year there is rain somewhere along their western aspect.

During the southern winter the cloud bank of the coast is best developed and precipitation is greatest. At Lima, for instance, the clear skies of March and April begin to be clouded in May, and the cloudiness grows until, from late June to September, the sun is invisible for weeks at a time. This is the period of the garua (mist) or the “tiempo de lomas,” the “season of the hills,” when the moisture clothes them with verdure and calls thither the herds of the coast valleys.

During the southern summer on account of the greater relative difference between the temperatures of land and water, the sea-breeze attains its maximum strength. It then accomplishes its greatest work in the desert. On the pampa of La Joya, for example, the sand dunes move most rapidly in the summer. According to the Peruvian Meteorological Records of the Harvard Astronomical Observatory the average movement of the dunes from April to September, 1900, was 1.4 inches per day, while during the summer months of the same year it was 2.7 inches. In close agreement are the figures for the wind force, the record for which also shows that 95 per cent of the winds with strength over 10 miles per hour blew from a southerly direction. Yet during this season the coast is generally clearest of fog and cloud. The explanation appears to lie in the exceedingly delicate nature of the adjustments between the various rain-making forces. The relative humidity of the air from the sea is always high, but on the immediate coast is slightly less so in summer than in winter. Thus in Mollendo the relative humidity during the winter of 1895 was 81 per cent; during the summer 78 per cent. Moreover, the temperature of the Coast Range is considerably higher in summer than in winter, and there is a tendency to reëvaporation of any moisture that may be blown against it. The immediate shore, indeed, may still be cloudy as is the case at Callao, which actually has its cloudiest season in the summer but the hills are comparatively clear. In consequence the sea-air passes over into the desert, where the relative increase in temperature has not been so great (compare Mollendo and La Joya in the curve for mean monthly temperature), with much higher vapor content than in winter. The relative humidity for the winter season at La Joya, 1895, was 42.5 per cent; for the summer season 57 per cent. The influence of the great barrier of the Maritime Cordillera, aided doubtless by convectional rising, causes ascent of the comparatively humid air and the formation of cloud. Farther eastward, as the topographic influence is more strongly felt, the cloudiness increases until on the border zone, about 8,000 feet in elevation, it may thicken to actual rain. Data have been selected to demonstrate this eastern gradation of meteorological phenomena.

At La Joya, a station on the desert northeast of Mollendo at an elevation of 4,140 feet, cloudiness is always slight, but it increases markedly during the summer. Caraveli, at an altitude of 5,635 feet,[22] and near the eastern border of the pampa, exhibits a tendency toward the climatic characteristics of the adjacent zone. Data for a camp station out on the pampa a few leagues from the town, were collected by Mr. J. P. Little of the staff of the Peruvian Expedition of 1912-13. They relate to the period January to March, 1913. Wind roses for these months show the characteristic light northwesterly winds of the early morning hours, in sharp contrast with the strong south and southwesterly indraught of the afternoon. The daily march of cloudiness is closely coördinated. Quotations from Mr. Little’s field notes follow:

“In the morning there is seldom any noticeable wind. A breeze starts at 10 A. M., generally about 180° (i. e. due south), increases to 2 or 3 velocity at noon, having veered some 25° to the southwest. It reaches a maximum velocity of 3 to 4 at about 4.00 P. M., now coming about 225° (i. e. southwest). By 6 P. M. the wind has died down considerably and the evenings are entirely free from it. The wind action is about the same every day. It is not a cold wind and, except with the fog, not a damp one, for I have not worn a coat in it for three weeks. It has a free unobstructed sweep across fairly level pampas.... At an interval of every three or four days a dense fog sweeps up from the southwest, dense enough for one to be easily lost in it. It seldom makes even a sprinkle of rain, but carries heavy moisture and will wet a man on horseback in 10 minutes. It starts about 3 P.M. and clears away by 8.00 P. M..... During January, rain fell in camp twice on successive days, starting at 3.00 P. M. and ceasing at 8.00 P. M. It was merely a light, steady rain, more the outcome of a dense fog than a rain-cloud of quick approach. In Caraveli, itself, I am told that it rains off and on all during the month in short, light showers.” This record is dated early in February and, in later notes, that month and March are recorded rainless.

Chosica (elevation 6,600 feet), one of the meteorological stations of the Harvard Astronomical Observatory, is still nearer the border. It also lies farther north, approximately in the latitude of Lima, and this in part may help to explain the greater cloudiness and rainfall. The rainfall for the year 1889-1890 was 6.14 inches, of which 3.94 fell in February. During the winter months when the principal wind observations were taken, over 90 per cent showed noon winds from a southerly direction while in the early morning northerly winds were frequent. It is also noteworthy that the “directions of the upper currents of the atmosphere as recorded by the motion of the clouds was generally between N. and E.” Plainly we are in the border region where climatic influences are carried over from the plateau and combine their effects with those from Pacific sources. Arequipa, farther south, and at an altitude of 7,550 feet, resembles Chosica. For the years 1892 to 1895 its mean rainfall was 5.4 inches.

Besides the seasonal variations of precipitation there are longer periodic variations that are of critical importance on the Coast Range. At times of rather regular recurrence, rains that are heavy and general fall there. Every six or eight years is said to be a period of rain, but the rains are also said to occur sometimes at intervals of four years or ten years. The regularity is only approximate. The years of heaviest rain are commonly associated with an unusual frequency of winds from the north, and an abnormal development of the warm current, El Niño, from the Gulf of Guayaquil. Such was the case in the phenomenally rainy year of 1891. The connection is obscure, but undoubtedly exists.

The effects of the heavy rains are amazing and appear the more so because of the extreme aridity of the country east of them. During the winter the desert traveler finds the air temperature rising to uncomfortable levels. Vegetation of any sort may be completely lacking. As he approaches the leeward slope of the Coast Range, a cloud mantle full of refreshing promise may be seen just peeping over the crest (Fig. 91). Long, slender cloud filaments project eastward over the margin of the desert. They are traveling rapidly but they never advance far over the hot wastes, for their eastern margins are constantly undergoing evaporation. At times the top of the cloud bank rises well above the crest of the Coast Range, and it seems to the man from the temperate zone as if a great thunderstorm were rising in the west. But for all their menace of wind and rain the clouds never get beyond the desert outposts. In the summer season the aspect changes, the heavy yellow sky of the desert displaces the murk of the coastal mountains and the bordering sea.

It is an age-old strife renewed every year and limited to a narrow field of action, wonderfully easy to observe. We saw it in its most striking form at the end of the winter season in October, 1911, and for more than a day watched the dark clouds rise ominously only to melt into nothing where the desert holds sway. At night we camped beside a scum-coated pool of alkali water no larger than a wash basin. It lay in a valley that headed in the Coast Range, and carried down into the desert a mere trickle that seeped through the gravels of the valley floor. A little below the pool the valley cuts through a mass of granite and becomes a steep-walled gorge. The bottom is clogged with waste, here boulders, there masses of both coarse and fine alluvium. The water in the valley was quite incapable of accomplishing any work except that associated with solution and seepage, and we saw it in the wet season of an unusually wet year. Clearly there has been a diminution in the water supply. But time prevented us from exploring this particular valley to its head, to see if the reduction were due to a change of climate, or only to capture of the head-waters by the vigorous rain-fed streams that enjoy a favorable position on the wet seaward slopes and that are extending their watershed aggressively toward the east at the expense of their feeble competitors in the dry belt.

An early morning start enabled me to witness the whole series of changes between the clear night and the murky day, and to pass in twelve hours from the dry desert belt through the wet belt, and emerge again into the sunlit terraces at the western foot of the Coast Range. Two hours before daylight a fog descended from the hills and the going seemed to be curiously heavy for the beasts. At daybreak my astonishment was great to find that it was due to the distinctly moist sand. We were still in the desert. There was not a sign of a bush or a blade of grass. Still, the surface layer, from a half inch to an inch thick, was really wet. The fog that overhung the trail lifted just before sunrise, and at the first touch of the sun melted away as swiftly as it had come. With it went the surface moisture and an hour after sunrise the dust was once more rising in clouds around us.

We had no more than broken camp that morning when a merchant with a pack-train passed us, and shouted above the bells of the leading animals that we ought to hurry or we should get caught in the rain at the pass. My guide, who, like many of his kind, had never before been over the route he pretended to know, asked him in heaven’s name what drink in distant Camaná whence he had come produced such astonishing effects as to make a man talk about rain in a parched desert. We all fell to laughing and at our banter the stranger stopped his pack-train and earnestly urged us to hurry, for, he said, the rains beyond the pass were exceptionally heavy this year. We rode on in a doubtful state of mind. I had heard about the rains, but I could not believe that they fell in real showers!

About noon the cloud bank darkened and overhung the border of the desert. Still the sky above us was clear. Then happened what I can yet scarcely believe. We rode into the head of a tiny valley that had cut right across the coast chain. A wisp of cloud, an outlier of the main bank, lay directly ahead of us. There were grass and bushes not a half-mile below the bare dry spot on which we stood. We were riding down toward them when of a sudden the wind freshened and the cloud wisp enveloped us, shutting out the view, and ten minutes later the moisture had gathered in little beads on the manes of our beasts and the trail became slippery. In a half-hour it was raining and in an hour we were in the midst of a heavy downpour. We stopped and pastured our famished beasts in luxuriant clover. While they gorged themselves a herd of cattle drifted along, and a startled band of burros that suddenly confronted our beasts scampered out of sight in the heavy mist. Later we passed a herdsman’s hut and long before we reached him he shouted to us to alter our course, for just ahead the old trail was wet and treacherous at this time of year. The warning came too late. Several of our beasts lost their footing and half rolled, half slid, down hill. One turned completely over, pack and all, and lay in the soft mud calmly taking advantage of the delay to pluck a few additional mouthfuls of grass. We were glad to reach firmer ground on the other side of the valley.

The herdsmen were a hospitable lot. They had come from Camaná and rarely saw travelers. Their single-roomed hut was mired so deeply that one found it hard to decide whether to take shelter from the rain inside or escape the mud by standing in the rain outside. They made a little so-called cheese, rounded up and counted the cattle on clear days, drove them to the springs from time to time, and talked incessantly of the wretched rains in the hills and the delights of dry Camaná down on the coast. We could not believe that only some hours’ traveling separated two localities so wholly unlike.

The heavy showers and luxuriant pastures of the wet years and the light local rains of the dry years endow the Coast Range with many peculiar geographic qualities. The heavy rains provide the desert people at the foot of the mountains such a wealth of pasture for their burdensome stock as many oases dwellers possess only in their dreams. From near and far cattle are driven to the wet hill meadows. Some are even brought in from distant valleys by sea, yet only a very small part of the rich pastures can be used. It is safe to say that they could comfortably support ten times the number of cattle, mules, and burros that actually graze upon them. The grass would be cut for export if the weather were not so continually wet and if there were not so great a mixture of weeds, flowers, and shrubs.

Then come the dry years. The surplus stock is sold, and what remains is always maintained at great expense. In 1907 I saw stock grazing in a small patch of dried vegetation back of Mollendo, although they had to be driven several miles to water. They looked as if they were surviving with the greatest difficulty and their restless search for pasture was like the search of a desperate hunter of game. In 1911 the same tract was quite devoid of grass, and except for the contour-like trails that completely covered the hills no one would even guess that this had formerly been a cattle range. The same year, but five months later, a carpet of grass, bathed in heavy mist, covered the soil; a trickle of water had collected in pools on the valley floor; several happy families from the town had laid out a prosperous-looking garden; there were romping children who showed me where to pick up the trail to the port; on every hand was life and activity because the rains had returned bringing plenty in their train. I asked a native how often he was prosperous.

“Segun el temporal y la Providencia” (according to the weather and to Providence), he replied, as he pointed significantly to the pretty green hills crowned with gray mist.

It, therefore, seems fortunate that the Coast Range is so placed as to intercept and concentrate a part of the moisture that the sea-winds carry, and doubly fortunate that its location is but a few miles from the coast, thereby giving temporary relief to the relatively crowded people of the lower irrigated valleys and the towns. The wet years formerly developed a crop of prospectors. Pack animals are cheaper when there is good pasture and they are also easier to maintain. So when the rains came the hopeful pick-and-shovel amateurs began to emigrate from the towns to search for ore among the discolored bands of rock intruded into the granite masses of the coastal hills. However, the most likely spots have been so thoroughly and so unsuccessfully prospected for many years that there is no longer any interest in the “mines.”

Transportation rates are still most intimately related to the rains. My guide had two prices--a high price if I proposed to enter a town at night and thus require him to buy expensive forage; a low price if I camped in the hills and reached the town in time for him to return to the hills with his animals. Inquiry showed that this was the regular custom. I also learned that in packing goods from one part of the coast to another forage must be carried in dry years or the beasts required to do without. In wet years by a very slight detour the packer has his beasts in good pasture that is free for all. The merchant who dispatches the goods may find his charges nearly doubled in extremely dry years. Goods are more expensive and there is a decreased consumption. The effects of the rains are thus transmitted from one to another, until at last nearly all the members of a community are bearing a share of the burdens imposed by drought. As always there are a few who prosper in spite of the ill wind. If the pastures fail, live stock _must_ be sold and the dealers ship south to the nitrate ports or north to the large coast towns of Peru, where there is always a demand. Their business is most active when it is dry or rather at the beginning, of the dry period. Also if transport by land routes becomes too expensive the small traders turn to the sea routes and the carriers have an increased business. But so far as I have been able to learn, dry years favor only a few scattered individuals.

To the traveler on the west coast it is a source of constant surprise that the sky is so often overcast and the ports hidden by fog, while on every hand there are clear evidences of extreme aridity. Likewise it is often inquired why the sunsets there should be often so superlatively beautiful during the winter months when the coast is fog bound. Why a desert when the air is so humid? Why striking sunsets when so many of the days are marked by dull skies? As we have seen in the first part of this chapter, the big desert tracts lie east of the Coast Range, and there, excepting slight summer cloudiness, cloudless skies are the rule. The desert just back of the coast is in many parts of Peru only a narrow fringe of dry marine terraces quite unlike the real desert in type of weather and in resources. The fog bank overhanging it forms over the Humboldt Current which lies off shore; it drifts landward with the onshore wind; it forms over the upwelling cold water between the current and the shore; it gathers on the seaward slopes of the coastal hills as the inflowing air ascends them in its journey eastward. Sometimes it lies on the surface of the land and the water; more frequently it is some distance above them. On many parts of the coast its characteristic position is from 2,000 to 4,000 feet above sea level, descending at night nearly or quite to the surface, ascending by day and sometimes all but disappearing except as rain-clouds on the hills.[23] Upon the local behavior of the fog bank depends in large measure the local climate. A general description of the coastal climate will have many exceptions. The physical principles involved are, however, the same everywhere. I take for discussion therefore the case illustrated by Fig. 92, since this also displays with reasonable fidelity the conditions along that part of the Peruvian coast between Camaná and Mollendo which lies in the field of work of the Yale Peruvian Expedition of 1911.

Three typical positions of the fog bank are shown in the figure, and a fourth--that in which the bank extends indefinitely westward--may be supplied by the imagination.

If the cloud bank be limited to _C_ only the early morning hours at the port are cloudy. If it extend to _B_ the sun is obscured until midday. If it reach as far west as _A_ only a few late afternoon hours are sunny. Once in a while there is a sudden splash of rain--a few drops which astonish the traveler who looks out upon a parched landscape. The smaller drops are evaporated before reaching the earth. In spite of the ever-present threat of rain the coast is extremely arid. Though the vegetation appears to be dried and burned up, the air is humid and for months the sky may be overcast most of the time. So nicely are the rain-making conditions balanced that if one of our ordinary low-pressure areas, or so-called cyclonic storms, from the temperate zone were set in motion along the foot of the mountains, the resulting deluge would immediately lay the coast in ruins. The cane-thatched, mud-walled huts and houses would crumble in the heavy rain like a child’s sand pile before a rising sea; the alluvial valley land would be coated with infertile gravel; and mighty rivers of sand, now delicately poised on arid slopes, would inundate large tracts of fertile soil.

If the fog and cloud bank extend westward indefinitely, the entire day may be overcast or the sun appear for a few moments only through occasional rifts. Generally, also, it will make an appearance just before sunset, its red disk completely filling the narrow space between the under surface of the clouds and the water. I have repeatedly seen the ship’s passengers and even the crew leave the dinner table and collect in wondering groups about the port-holes and doorways the better to see the marvelous play of colors between sky and sea. It is impossible not to be profoundly moved by so majestic a scene. A long resplendent path of light upon the water is reflected in the clouds. Each cloud margin is tinged with red and, as the sun sinks, the long parallel bands of light are shortened westward, changing in color as they go, until at last the full glory of the sunset is concentrated in a blazing arc of reds, yellows, and purples, that to most people quite atones for the dull gray day and its humid air.

At times the clouds are broken up by the winds and scattered helter-skelter through the west. A few of them may stray into the path of the sun temporarily to hide it and to reflect its primary colors when the sun reappears. From the main cloud masses there reach out slender wind-blown streamers, each one delicately lighted as the sun’s rays filter through its minute water particles. Many streamers are visible for only a short distance, but when the sun catches them their filmy invisible fingers become delicate bands of light, some of which rapidly grow out almost to the dome of the sky. Slowly they retreat and again disappear as the rays of the sun are gradually shut off by the upturning curve of the earth.

The unequal distribution of precipitation in the climatic zones of western Peru has important hydrographic consequences. These will now be considered. In the preceding figure four types of stream profiles are displayed and each has its particular relation to the cloud bank. Stream 1 is formed wholly upon the coastal terraces beneath the cloud bank. It came into existence only after the uplift of the earth’s crust that brought the wave-cut platforms above sea level. It is extremely youthful and on account first of the small seepage at its headquarters--it is elsewhere wholly without a tributary water supply--and, second, of the resistant granite that occurs along this part of the coast, it has very steep and irregular walls and an ungraded floor. Many of these “quebradas” are difficult to cross. A few of them have fences built across their floors to prevent the escape of cattle and burros that wander down from the grassy hills into the desert zone. Others are partitioned off into corrals by stone fences, the steep walls of the gorge preventing the escape of the cattle. To these are driven the market cattle, or mules and burros that are required for relays along the shore trail.

Stream 2 heads in the belt of rains. Furthermore it is a much older stream than 1, since it dates back to the time when the Coast Range was first formed. It has ample tributary slopes and a large number of small valleys. A trickle of water flows down to become lost in the alluvium of the lower part of the valley or to reappear in scattered springs. Where springs and seepage occur together, an olive grove or a garden marks the spot, a corral or two and a mud or stone or reed hut is near by, and there is a tiny oasis. Some of these dots of verdure become so dry during a prolonged drought that the people, long-established, move away. To others the people return periodically. Still others support permanent settlements.

Stream 3 has still greater age. Its only competitors are the feeble, almost negligible, streams that at long intervals flow east toward the dry zone. Hence it has cut back until it now heads in the desert. Its widely branched tributaries gather moisture from large tracts. There is running water in the valley floor even down in the terrace zone. At least there are many dependable springs and the permanent homes that they always encourage. A valley of this type is always marked by a well-defined trail that leads from settlement to settlement and eastward over the “pass” to the desert and the Andean towns.

Stream 4 is a so-called “antecedent” stream. It existed before the Coast Range was uplifted and cut its channel downward as the mountains rose in its path. The stretch where it crosses the mountains may be a canyon with a narrow, rocky, and uncultivable floor, so that the valley trails rise to a pass like that at the head of stream 3, and descend again to the settlements at the mouth of 4. There is in this last type an abundance of water, for the sources of the stream are in the zone of permanent snows and frequent winter rains of the lofty Cordillera of the Andes. The settlements along this stream are continuous, except where shut-ins occur--narrow, rocky defiles caused by more resistant rock masses in the path of the stream. Here and there are villages. The streams have fish. When the water rises the river may be unfordable and people on opposite sides must resort to boats or rafts.[24]

EASTERN BORDER CLIMATES

On windward mountain slopes there is always a belt of maximum precipitation whose elevation and width vary with the strength of the wind, with the temperature, and with the topography. A strong and constant wind will produce a much more marked concentration of the rainfall. The belt is at a low elevation in high latitudes and at a high elevation in low latitudes, with many irregularities of position dependent upon the local and especially the minimum winter temperature. The topographic controls are important, since the rain-compelling elevation may scatter widely the localities of maximum precipitation or concentrate them within extremely narrow limits. The human effects of these climatic conditions are manifold. Wherever the heaviest rains are, there, too, as a rule, are the densest forests and often the most valuable kinds of trees. If the general climate be favorable and the region lie near dense and advanced populations, exploitation of the forest and progress of the people will go hand in hand. If the region be remote and some or all of the people in a primitive state, the forest may hinder communication and retard development, especially if it lie in a hot zone where the natural growth of population is slow.... These are some of the considerations we shall keep in mind while investigating the climate of the eastern border of the Peruvian Andes.

The belt of maximum precipitation on the eastern border of the Andean Cordillera in Peru lies between 4,000 and 10,000 feet. Judging by the temporary records of the expedition and especially by the types of forest growth, the heaviest rains occur around 8,000 feet. It is between these elevations that the densest part of the Peruvian _montaña_ (forest) is found. The cold timber line is at 10,500 feet with exceptional extensions of a few species to 12,500 feet. In basins or deep secluded valleys near the mountain border, a dry timber line occurs at 3,000 feet with many variations in elevation due to the variable declivity and exposure of the slopes and degree of seclusion of the valleys. Elsewhere, the mountain forest passes without a break into the plains forest with change in type but with little change in density. The procumbent and suppressed trees of the cold timber line in regions of heavy winter snows are here absent, for the snows rarely reach below 14,000 feet and even at that elevation they are only light and temporary. The line of perpetual snow is at 15,000 feet. This permanent gap of several thousand feet vertical elevation between the zone of snow and the zone of forest permits the full extension of many pioneer forest species, which is to say, there is an irregular development of the cold timber line. It also permits the full use of the pasture belt above the timber (Fig. 97), hence permanent habitations exist but little below the snowline and a group of distinctive high-mountain folk enjoys a wide distribution. There is a seasonal migration here, but it is not wholesale; there are pastures snow-covered in the southern winter, but, instead of the complete winter burial of the Alpine meadows of our western mountains, we have here only a buried upper fringe. All the rest of the pasture belt is open for stock the year round.

This climatic distinction between the lofty grazing lands of the tropics and those of the temperate zones is far-reaching. Our mountain forests are not utilized from above but from below. Furthermore, the chief ways of communication lead around our forests, or, if through them, only for the purpose of putting one population group in closer touch with another. In the Peruvian Andes the largest population groups live above the forest, not below it or within it. It must be and is exploited from above.

Hence railways to the eastern valleys of Peru have two chief objects, (1) to get the plantation product to the dense populations above the forest and (2) to bring timber from the _montaña_ to the treeless plateau. The mountain prospector is always near a habitation; the rubber prospector goes down into the forested valleys and plains far from habitations. The forest separates the navigable streams from the chief towns of the plateau; it does not lead down to rich and densely populated valley floors.

Students in eastern Peru should find it a little difficult to understand poetical allusions to silent and lonely highlands in contrast to the busy life of the valleys. To them Shelley’s description of the view from the Euganean Hills of northern Italy,

“Beneath is spread like a green sea The waveless plain of Lombardy, ... Islanded by cities fair,”

might well seem to refer to a world that is upside down.

There is much variation in the forest types between the mountains and the plains. At the top of the forest zone the warm sunny slopes have a forest cover; the shady slopes are treeless. At the lower edge of the grassland, only the shady slopes are forested (Fig. 53B). Cacti of arboreal size and form grow on the lofty mountains far above the limits of the true forest; they also appear at 3,000 feet in modified form, large, rank, soft-spined, and in dense stands on the semi-arid valley floors below the dry timber line. Large tracts between 8,000 and 10,000 feet are covered with a forest growth distributed by species--here a dense stand of one type of tree, there another. This is the most accessible part of the Peruvian forest and along the larger valleys it is utilized to some extent. The number of species is more limited, however, and the best timber trees are lower down. Though often referred to as jungle, the lowlier growths at the upper edge of the forest zone have no resemblance to the true jungle that crowds the lowland forest. They are merely an undergrowth, generally open, though in some places dense. They are nowhere more dense than many examples from New England or the West.

Where deep valleys occur near the border of the mountains there is a semi-arid climate below and a wet climate above, with a correspondingly greater number of species within short distances of each other. This is a far more varied forest than at the upper edge of the timber zone or down on the monotonous plains. It has a higher intrinsic value than any other. That part of it between the Pongo and Yavero (1,200 to 4,000 feet) is very beautiful, with little undergrowth except a light ground-cover of ferns. The trees are from 40 to 100 feet in height with an average diameter of about 15 inches. It would yield from 3,000 to 5,000 board feet per acre exclusive of the palms. There are very few vines suspended from the forest crown and the trunks run clear from 30 to 60 feet above the ground. Were there plenty of labor and a good transportation line, these stands would have high economic value. Among the most noteworthy trees are the soft white cedar, strong and light; the amarillo and the sumbayllo, very durable in water; the black nogal, and the black balsam, straight and easy to work; the heavy yunquero, which turns pink when dry; the chunta or black palm, so hard and straight and easy to split that wooden nails are made from it; and the rarer sandy matico, highly prized for dug-out canoes. Also from the chunta palm, hollow except for a few central fibers, easily removed, pipes are made to convey water. The cocobolo has a rich brown color and a glossy surface and is very rare, hence is much sought after for use in furniture making. Most of these woods take a brilliant polish and exhibit a richness and depth of color and a beauty of grain that are rare among our northern woods.

The plains forest northeast of the mountains is in the zone of moderate rainfall where there is one long dry season and one long wet season. When it is dry the daytime temperatures rise rapidly to such high levels that the relative humidity of the air falls below 50 per cent (Fig. 110). The effect on the vegetation is so marked that many plants pass into a distinctly wilted condition. On clear days the rapid fall in the relative humidity is astonishing. By contrast the air on the mountain border heats more slowly and has a higher relative humidity, because clouds form almost constantly in the ascending air currents and reflect and absorb a large part of the heat of the sun’s rays. It is striking to find large tracts of cane and bamboo on the sand bars and on wet shady hillslopes in the slope belt, and to pass out of them in going to the plains with which we generally associate a swamp vegetation. They exist on the plains, but only in favored, that is to say wet, spots. Larger and more typical tracts grow farther north where the heavier rains of the Amazon basin fall.

The floods of the wet tropical season also have a restricting influence upon the tropical forest. They deliver such vast quantities of water to the low-gradient lowland streams that the plains rivers double, even treble, their width and huge pools and even temporary lakes form in the shallow depressions back of the natural levees. Of trees in the flooded areas there are only those few species that can grow standing in water several months each year. There are also cane and bamboo, ferns in unlimited numbers, and a dense growth of jungle. These are the haunts of the peccary, the red forest deer, and the jungle cat. Except along the narrow and tortuous animal trails the country is quite impassable. Thus for the sturdiest and most useful forest growth the one-wet-one-dry season zone of the plains has alternately too much and too little water. The rubber tree is most tolerant toward these conditions. Some of the best stands of rubber trees in Amazonia are in the southwestern part of the basin of eastern Peru and Bolivia, where there is the most typical development of the habitat marked by the seasonal alternation of floods and high temperatures.

When tropical agriculture is extended to the plains the long dry season will be found greatly to favor it. The southwestern quadrant of the Amazon basin, above referred to, is the best agricultural area within it. The northern limits of the tract are only a little beyond the Pongo. Thence northward the climate becomes wetter. Indeed the best tracts of all extend from Bolivia only a little way into southeastern Peru, and are coincident with the patchy grasslands that are there interspersed with belts of woodland and forest. Sugar-cane is favored by a climate that permits rapid growth with a heavy rainfall and a dry season is required for quality and for the harvest. Rice and a multitude of vegetable crops are also well suited to this type of climate. Even corn can be grown in large quantities.

At the present time tropical agriculture is almost wholly confined to the mountain valleys. The reasons are not wholly climatic, as the above enumeration of the advantages of the plains suggests. The consuming centers are on the plateau toward the west and limitation to mule pack transport always makes distance in a rough country a very serious problem. The valleys combine with the advantage of a short haul a climate astonishingly like the one just described. In fact it is even more extreme in its seasonal contrasts. The explanation is dependent upon precisely the same principles we have hitherto employed. The front range of the Andes and the course of the Urubamba run parallel for some distance. Further, the front range is in many places somewhat higher than the mountain spurs and knobs directly behind it. Even when these relations are reversed the front range still acts as a barrier to the rains for all the deep valleys behind it whose courses are not directly toward the plains. Thus, one of the largest valleys in Peru, the Urubamba, drops to 3,400 feet at Santa Ana and to 2,000 feet at Rosalina, well within the eastern scarp of the Andes. The mountains immediately about it are from 6,000 to 10,000 feet high. The result is a deep semi-arid pocket with only a patchy forest (Fig. 54, p. 79).[25] In places the degree of seclusion from the wind is so great that the scrub, cacti, and irrigation remind one strongly of the desert on the border of an oasis, only here the transition is toward forests instead of barren wastes. The dense forest, or _montaña_, grows in the zone of clouds and maximum precipitation between 4,000 and 10,000 feet. At the lower limit it descends a thousand feet farther on shady slopes than it does on sunny slopes. The continuous forest is so closely restricted to the cloud belt that in Fig. 99 the two limits may be seen in one photograph. All these sharply defined limits and contrasts are due to the fact that the broad valley, discharging through a narrow and remote gorge, is really to leeward of all the mountains around it. It is like a real desert basin except in a lesser degree of exclusion from the rains. If it were narrow and small the rains formed on the surrounding heights would be carried over into it. Rain on the hills and sunshine in the valley is actually the day-by-day weather of the dry season. In the wet season the sky is overcast, the rains are general, though lighter in the valley pocket, and plants there have then their season of most rapid growth. The dry season brings plants to maturity and is the time of harvest. Hence sugar and cacao plantations on a large scale, hence a varied life in a restricted area, hence a distinct geographic province unique in South America.

INTER-ANDEAN VALLEY CLIMATES

Not all the deep Andean valleys lie on or near the eastern border. Some, like the Apurimac and the Marañon, extend well into the interior of the Cordillera. Besides these deep remote valleys with their distinct climatic belts are basins, most of them with outlets to the sea--broad structural depressions occurring in some cases along large and in others along small drainage lines. The Cuzco basin at 11,000 feet and the Abancay basin at 6,000 to 8,000 feet are typical. Both have abrupt borders, narrow outlets, large bordering alluvial fans, and fertile irrigable soil. Their difference of elevation occurs at a critical level. Corn will ripen in the Cuzco basin, but cane will not. Barley, wheat, and potatoes are the staple crops in the one; sugar-cane, alfalfa, and fruit in the other. Since both are bordered by high pastures and by mineralized rocks, the deeper Abancay basin is more varied. If it were not so difficult to get its products to market by reason of its inaccessibility, the Abancay basin would be the more important. In both areas there is less rainfall on the basin floor than on the surrounding hills and mountains, and irrigation is practised, but the deeper drier basin is the more dependent upon it. Many small high basins are only within the limits of potato cultivation. They also receive proportionately more rain. Hence irrigation is unnecessary. According as the various basins take in one or another of the different product levels (Fig. 35) their life is meager and unimportant or rich and interesting.

The deep-valley type of climate has the basin factors more strongly developed. Below the Canyon of Choqquequirau, a topographic feature comparable with the Canyon of Torontoy, the Apurimac descends to 3,000 feet, broadens to several miles, and has large alluvial fans built into it. Its floor is really arid, with naked gravel and rock, cacti stands, and gnarled shrubs as the chief elements of the landscape. Moreover the lower part of the valley is the steeper. A former erosion level is indicated in Fig. 125. When it was in existence the slopes were more moderate than now and the valley broad and open. Thereupon came uplift and the incision of the stream to its present level. As a result, a steep canyon was cut in the floor of a mature valley. Hence the slopes are in a relation unlike that of most of the slopes in our most familiar landscapes. The gentle slopes are above, the steep below. The break between the two, a topographic unconformity, may be distinctly traced.

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.