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The climate of Peru varies, ranging
from tropical in the montañas to arctic in the highest
mountains of the Andes. Average temperatures decrease
about 1.7 Celsius degrees (about 3 Fahrenheit degrees)
with every 450-m (1,500-ft) increase in elevation.
Permanent snow and ice fields cover peaks more than
5,000 m (16,500 ft) above sea level, and the highest
elevation at which the land is suitable for agriculture
is about 4,400 m (14,500 ft).
In the coastal plain, the temperature is normally mild,
averaging about 20° C (about 68° F) throughout the year.
The coastal climate is moderated by winds blowing from
the cool offshore current known as the Peru, or
Humboldt, Current. The coast receives less than 50 mm (less
than 2 in) of precipitation each year, largely because
the cordilleras receive most of the rain carried by the
trade winds from the east. Mist-laden clouds known as
garúa shroud many of the slopes of the sierra from June
to October, providing enough moisture to support
grasslands.
In the sierra, the temperature ranges seasonally from
about -7° to 21° C (about 20° to 70° F). Rainfall is
usually scanty, but in some localities heavy rains fall
from October to April. In Cusco, in the southeastern
sierra, annual rainfall averages some 815 mm (some 32
in). The exposed eastern slopes of the Andes receive
more than 2,500 mm (100 in) of rain annually, but
sheltered locations receive much less. Rainfall amounts
diminish rapidly southward, causing many changes in the
vegetation.
The montaña region is extremely hot and humid, although
at higher altitudes it is less so. The prevailing
easterly winds blowing across that region gather
moisture that is later deposited on the eastern Andean
slopes. Annual rainfall in some districts averages as
much as 3,810 mm (as much as 150 in). Most of this rain,
which principally falls from November through April,
eventually drains back to the montaña.
Peru’s climate periodically experiences a weather
pattern known as El Niño. El Niño occurs every three to
seven years when unusually warm ocean conditions appear
along the western coast. During El Niño the wet weather
conditions normally present in the western Pacific move
to the east, bringing heavy rains that can cause
extensive flooding.
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