Peru is about twice the size of France or the state of Texas, extending between the equator and 18°S. It has a long coastline on the Pacific Ocean and is bordered by Ecuador and Colombia on the north, Brazil and Bolivia on the east, and Chile on the south. The central portion of Peru includes the great mountain and plateau region of the Andes, with numerous peaks rising to over 6,000 m/20,000 ft and with extensive plateaux districts between 3,000 m/10,000 ft and 4,300 m/14,000 ft. There is a very narrow coastal plain on the Pacific shore, while to the east of the Andes, the land drops steeply to the forested lowlands of the Amazon basin.
The Pacific coastal district has a most unusual type of dry desert climate. This is caused by the cold waters of the Peru Current, which flows northwards. This area is a continuation of the coastal desert of northern Chile. The cold ocean water maintains low temperatures for a tropical latitude almost up to the equator and there are very small differences from month to month. The dryness is so marked that in some places several years have passed without appreciable rain. In the northern coastal districts, however, there is a remarkable change of weather for a few weeks every ten or fifteen years. Temperature rises, clouds build up, and torrential rain may fall for many days. It is as if the equatorial belt of cloud and rain, which normally lies to the north on the coasts of Ecuador and Colombia, had moved south. Such unusual and unexpected heavy rain may cause widespread damage. At the same time the sea temperature offshore rises and the cold current retreats southwards. The phenomenon is called El Niño, and is most likely to occur in December and January. This otherwise arid coastal strip experiences frequent low cloud and fog from which a light drizzle, called locally garúa, may fall. This is another unusual feature for such a dry climate. The climatic table for Lima is representative of the climate of the coastal district although the city is a short distance inland. Midday temperatures are here a little higher than on the coast. Lima has an average of only one to two hours of sunshine a day in the low-sun period, but this rises to between five to seven hours a day during the warmer months of December to April.
The Andean mountain and plateaux region of Peru has similar weather and climate to that described for the Andean region of Bolivia. Here the main differences are a consequence of the altitude. The tables for Cajamarca in the north of the country and Cusco in the south illustrate the marked reduction of temperature in all months and the single rainy season at the time of high sun between November and March. The greater cloudiness during the rainy season prevents the temperature from rising higher at this time. As another consequence of the high altitude there is a large daily range of temperature which falls quite low at night. During the dry season frosts may be a nightly occurrence at these heights. Above 3,000 m/10,000 ft visitors may suffer from mountain sickness as described for Bolivia.
The climate and weather of the eastern lowlands in the Peruvian portion of the Amazon basin are similar to those described for Brazil and Bolivia; they are illustrated by the tables for Concepción in Bolivia and Sena Madureira in Brazil.
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