Throughout the length of the Peruvian Andes there are 27 practicable passes, most of which are over 12,000 ft. high. The highest in use, between Antabamba and Cotahuasi, is 17,400 ft., the lowest in the far north at Huarmaca (Piura), 7,085 ft. high.
Montairia.—The heavy precipitation on the eastern slopes of the eastern cordillera results in some of the profoundest erosion in progress to-day. The soil on the valley bottoms is a rich, alluvial fill, adapted to tropical crops. Sides of lower valleys, as well as slopes of mountains and foot-hills, are covered with impenetrable forest shrouded in mist.
The steaming equatorial basin below is an endless blue-green lowland with ranges of low hills, and here and there a glimpse of the windings of shining rivers which disappear in the mists of the horizon. These plains at the: foot of the Cordillera Oriental are less than ',coo ft. above the sea, usually much less. Topo graphically Amazonia is monotonous but botanically it is most diversified.
Climate.—Though there are three major climatic controls in Peru, latitude, ocean and the Cordillera de los Andes, the last is by far the most important. Change of climate is sought not by travelling north and south, but up and down, from west to east. A frigid zone can be reached in a few hours of ascent from tropical lowlands. Rainfall, which is heavy on the east, increases up to a certain height, decreasing from east to west across the sierra. The south-east trades sweep across the warm lowlands of Amazonia, filled with moisture. Confronted by the Andes, they are forced to rise, are chilled and heavy rains result. The annual rainfall on the lower eastern slopes of the Peruvian Andes is I50 in. or more. Swollen streams give rise to the world's greatest river-system. The trades cross the mountains and in descending do not again reach sea-level until 5o to 100 m. off shore. The Humboldt or Peruvian current, a mass of cold water about 15o m. wide, sweeping up from the south along the coast brings cool, moist south and south-west winds, which, meeting the warm air over the land, cause fogs (garrias) along the southern coast. They are about 1,500 ft. thick and extend about 5 to 15 m. in land. North of 8° S. there are none whatever, but a summer rain fall of a few inches occurs every few years, as in 1925. There are no electrical storms on the coast.
Climatic provinces are distinguished chiefly by variations in rainfall. From east to west, montana and ceja de la montana have precipitation throughout the year, heaviest in summer. Sierra rainfall, confined to summer, diminishes both in quantity and in length of season toward the south. The rainfall of the higher western slopes resembles that of the sierra, the amount of precipitation diminishing with altitude, disappearing entirely at altitudes which vary with latitude and exposure. (I) The immedi ate coast zone, though desert, has a cool, damp climate with small annual and diurnal variability. Mean annual temperatures at the north average about io° F warmer than those at the south (Paita F, Mollendo 65° F). The western slopes below 4 to 6,000 ft., though averaging about the same, have greater diurnal ranges of temperature. This uniformity, interrupted by no storms and no sudden changes of temperature, is very monotonous, especially since the skies, as far north as 8° S., are covered with clouds during more than half the year, though the total annual rainfall is under 2 inches. (2) The summer-rain zone is an extremely complex province. It includes, besides the western slopes above the rainless zone already described, the summit region above 15,000 ft., desolate, high plateaux, intermont basins and deep canyons, whose climates range from equatorial to frigid. Climates in Peru depend primarily on altitude; deep valleys are tropical to semi-tropical, depending on depth, valley-trend, slope and ex posure. Mean annual mina temperature averages about so° F, with greater diurnal than annual range. With an average relative humidity of 5o%, the mean annual rainfall is 25 in. at the south, where it is confined to summer, reaching 4o in. in the north, where precipitation occurs in spring, summer and autumn, over lapping the equatorial rainfall type. Snowfall occurs south of about 7° S. above 12,000 to 13,000 feet. Because of aridity the snow-line is very high. Heat in deep sierra valleys is so great and rainfall so slight, that often the narrow floors are semi-desert; while 4,000 to 6,000 ft. above those same floors rainfall may ex ceed 8o inches. Similarly, high-level basin stations have less rain fall than surrounding mountains, and irrigation is often necessary.