According to findjobdescriptions, scarce is the material that one possesses for the knowledge of the climate of Peru, being very few and very unevenly distributed the stations for which meteorological observations are made available for a conspicuous series of years. Five main climatic zones can be distinguished in bulk: the coastal region, the western slopes of the Sierra, the plateau, the eastern slopes of the Sierra, the Montaña. The coastal region has a peculiar oceanic desert climate (which E. De Martonne in his classification calls the Peruvian climate), characterized by strong relative humidity of the air (Callao 84%, Lima 82%), extremely scarce rainfall (Chiclayo, 17 mm. annui, Trujillo 30, Lima 48; Mollendo 22), frequent fogs, said garúas, rather uniform temperatures (Chiclayo, at 6 ° 43 ′ S., has an average annual temperature of 21 °, 0, with 17 °, 5 in the coldest month, June, and 25 °, 7 in the hottest month, February ; Trujillo, at 8 ° 5 ′, has respectively 20 °, 7, 17 °, 2 in June, and 25 °, 1 in January; Lima, at 12 ° 4 ′. Has 19 °, 3, 16 °, 1 in August, 23 °, 5 in February; Mollendo, at 1 °, 1 ′, has 18 °, 3, 15 °, 2 in August and 210.5 in February) and relatively low (negative anomaly due to the influence of the cold current of Peru, which flows along the coast towards S.-N.; Bahia, in Brazil, on the Atlantic coast and almost at the same latitude as Lima, has an average annual temperature of 24 °, 8). Precipitation falls in the summer months in the northern section of the coast, in the winter months in the center and south.
On the western slopes of the Sierra one passes, with increasing height, from the climate of the coast to that of the plateau. As you go up, as is well understood, temperatures decrease, higher in central Peru than in northern Peru (here the dense fogs, particularly thick between 2500 and 3000 m., Fed by the masses of steam coming from the Pacific without find the screen of coastal reliefs, as occurs in central and southern Peru, and the considerable rains that still manage to reach it from the east, negatively affect temperatures) and in the southern one (this section is very arid, so that the average temperatures are lowered by the strong nocturnal irradiation as well as by the highest latitude). Chosica, located at 2012 m. high on the Lima-Oroya railway, has 18 °, 2 of average annual temperature, 15 °, 7 in February and 19 °, 2 in September; rainfall (156 mm.), almost completely falls in the period February-April (2/3 in February); La Joya, on the Mollendo-Arequipa-Cuzco railway, at 1262 masl, has an annual average of 16 °, 9, 15 °, 3 in July and 18 °, 7 in February, and not eveni mm. of precipitation; Arequipa, at 2451 m., Has 13 °, 8 of annual average, 13 °, 1 in July and 14 °, 4 in September, and 106 mm. of precipitation, which falls for 90% in January-March.
A mountain climate prevails on the plateaus, similar to that of the high plateaus of Colombia and Ecuador, but with greater annual variations in temperatures and a dry period in the rainfall regime. Annual temperatures are usually below 15 ° even on the lower plateaus; the rains do not abound and fall mainly in summer; Caiamarca, at 6 ° 47 ′ S. and at 2810 msm, has an average annual temperature of 14 °, 6, 11 °, 9 in June, 15 °, 9 in December, 1144 mm. of precipitation (maximum in February, 234 mm.); Cuzco, at 13 ° 27 ′ and 3380 msm, has 10 °, 7 ° annual temperature, 8 °, 3 in July, 12 °, 1 in November, 804 mm. of precipitation (maximum in January, 162 mm.); Puno, on Lake Titicaca, at 15 ° 50 ′ and 3822 m., Has 8 °, 3, 5 °, 8, 10 °, 3, and 963 mm respectively. of precipitation (maximum in February, 206 mm.); finally Cerro de Pasco, at 10 ° 43 ′ and 4350 meters above sea level, has an average annual temperature of 5 °, 7, with 4 °, 7 in July and 6 °, 7 in January, and 885 mm. of precipitation (maximum in January, 117 mm.). Above 4400 m., On average, precipitation falls in a solid state.
On the eastern slopes of the Sierra we pass from the temperate climate to the subequatorial one and finally to the equatorial one. In Santa Ana, a village located at 1040 meters above sea level in the Urubamba valley, the average annual temperature is 22 °, 2, and no month drops below 20 °, the excursion between the hottest and coolest months is only 2 °, 2.
Overall temperatures drop from N. to S.; the nebulosity is strong, since this area is under the influence of the trade winds all year round, which pass over a humid lowland and are therefore full of steam, and the rains above 1800 m are abundant; further down, up to 1500 m. approximately, there is a less humid area, more accentuated in southern Peru than in northern Peru. The lower slopes of the Sierra have roughly the same climate as the Montaña, which is typically equatorial. Iquitos, in fact, at 3 ° 45 ′ S. and at 106 msm, has an annual temperature of 24 °, 8; 23 °, 4 ° in the coolest month, July, and 25 °, 8 ° in the hottest month, November. The rainfall amounts to 2623 mm. and are distributed in each month, none of which receives less than 100 mm. (minimum in August, 115 mm., maximum in March, 305).
The sanitary conditions are very different in the various parts of the country. In the coastal region, due to the shortage of drinking water, typhus and gastroenteritis are widespread; on the higher plateaus in the center and in the south, heart and respiratory organs are naturally widespread; in Montaña, the most unhealthy region, malaria, typhus and yellow fever are the diseases that cause the greatest number of victims. The climate of the lower plateaus is also excellent for the Whites.