Bolivia lies between 10° and 23°S in western South America. It is a landlocked country twice the size of France or the state of Texas. It is bordered on the north and east by Brazil, southeast by Paraguay, south by Argentina, and west by Chile and Peru.
The country is divided into two very contrasting physical regions which have very different weather and climate. In the west the great mountain range of the Andes, here at its broadest, rises to peaks of over 6,100 m/20,000 ft. Between the western and eastern ranges of the Andes there is an extensive highland plateau, the páramos or altiplano, at an altitude of 3,000-4,000 m/10,000-13,000 ft. East of the Andes the land drops sharply to the forested lowlands of the Amazon basin on the Brazilian border and the large lowland plain, known as the Gran Chaco, which extends through northern Paraguay to northern Argentina.
The climate of the Andean plateau is represented by the table for La Paz. Here there is an extreme type of tropical highland climate. Annual precipitation is low and occurs mostly between December and March, during the high-sun period when there are many rainy days. Temperatures are much reduced by altitude with only small differences from month to month. The daily range of temperatures is large, so the nights are quite cold, particularly during the dry winter or low-sun period when frost is almost a nightly occurrence.
In some parts of this region annual precipitation is as low as 250 mm/10 in. This is particularly so in the western mountains and valleys; consequently the permanent snowline may be as high as 6,100 m/20,000 ft. The high altitude of this region means that at 3,050 m/10,000 ft the atmospheric pressure is only about two-thirds that at sea level and at 5,200 m/17,000 ft it is only about half. This reduced pressure causes problems for visitors, who may suffer from mountain sickness, called here the soroche, on arrival. For a fit person acclimatization takes a few days to a week or so; but the area is not recommended for those who suffer from weak hearts or lung complaints.
Sunshine ranges from about six hours a day in the rainy season, when there is usually cloud in the afternoon, to as much as eight hours a day in the dry season. The sun's rays are particularly powerful because of the thin atmosphere at this height, and sunburn can be a hazard.
Temperature drops very rapidly after sunset and the nights feel distinctly chilly. The greatest differences of weather and climate in this high mountain region are those experienced during the course of the day and those arising from sudden changes of altitude.
The lowlands east of the Andes, comprising rather more than half the country, have a very different climate. Most of this region lies between 230 m/750 ft and 900 m/3,000 ft, and its weather and climate are similar to those found in the equatorial regions of the Amazon basin in Brazil or the Chaco region of Paraguay. Temperatures are warm to hot around the year, with a single rainy season at the time of high sun.
This region is much wetter than the Andean plateau, with the annual precipitation being everywhere more than 1,000 mm/40 in, rising to 1,500 mm/60 in towards the north. Here the combination of heat and humidity can cause discomfort and heat stress during the months October to March (see the table for Concepción).
During the dry period of low sun, occasional bursts of colder air reach these lowlands as a result of outbreaks of polar air from Antarctica. Temperatures may fall to a few degrees above freezing for a night or so. Such extreme events are rare, however.
In the Andean region of Bolivia, as elsewhere in Central and northern South America, the inhabitants distinguish three or four climatic zones depending on altitude: tierra caliente, from sea level to about 900 m/3,000 ft; tierra templada, between 900 m/3,000 ft and 1,800 m/6,000 ft; and tierra fria, from 1,800 m/6,000 ft to 3,000 m/10,000 ft, above which is the páramos or altiplano.
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