erly
winds from the Atlantic, and causes a very heavy rainfall on their
eastern slopes and over the forested Amazon plain. High temperatures
as well as excessive humidity prevail throughout this region. Farther
north, on the open llanos of the Orinoco tributaries, the year is
divided into equal parts, an alternating wet and dry season, the sun
temperatures being high followed by cool nights, and the temperatures
of the rainy season being even higher. The rainfall is heavy in the
wet season, causing many of the rivers to spread over extensive areas,
but in the dry season the inundated plains become dry, the large
rivers fed by the snows and rainfall of the Andes return within their
banks, the shallow lagoons and smaller streams dry up, vegetation
disappears, and the level plain becomes a desert. The northern plains
of the republic are swept by the north-east trades, and here, too, the
mountain barriers exercise a strongly modifying influence. The low
ridges of the Sierra de Perija do not wholly shut out these
moisture-laden winds, but they cause a heavy rainfall on their eastern
slopes, and create a dry area on their western flanks, of which the
Vale of Upar is an example. The higher masses of the Sierra Nevada de
Santa Marta cover a very limited area, leaving the trade winds a
comparatively unbroken sweep across the northern plains until checked
by the Western Cordillera, the Panama ranges and the Sierra de Baudo,
where a heavy precipitation follows. Farther south the coast ranges
cause a very heavy rainfall on their western slopes, which are quite
as uninhabitable because of rain and heat as are the coasts of
southern Chile through rain and cold. The rainfall on this coast is
said to average 73 in., though it is much higher at certain points and
in the Atrato Valley. As a result the coastal plain is covered with
swamps and tangled forests, and is extremely unhealthy, except at a
few favoured points on the coast. High temperatures prevail throughout
the greater part of the Magdalena and Cauca valleys, because the
mountain ranges which enclose them shut out the prevailing winds. At
Honda, on the Magdalena, 664 ft. above sea-level, the mean temperature
for the year is 82 deg. F., and the mercury frequently rises to 102
deg. in the shade. These lowland plains and valleys comprise the
climatic tropical zone of Colombia, which is characterized by high
tem