Not long ago, geologically speaking, Costa Rica did not exist. A mere
50 million years ago, this narrow mountainous strip of land was part
of the ocean floor, along with the rest of the Central American isthmus
between Guatemala and Colombia. Although that may sound like a long
time ago in earth's history, the dinosaurs had already disappeared
some 15 million years earlier.
Currently, most geologists believe that from 40 million years ago
until within only the past three million years what are now Costa
Rica and its neighboring countries were nothing more than a volcanic
archipelago. Using the theory of plate tectonics, which suggests that
the earth's crust is fragmented in a number of sections that fit together
like pieces of a jigsaw puzzle, though they are not stationary, the
formation of Costa Rica's land mass is explained by its position at
the western edge of the so-called Caribbean plate, which is moving
westward and overriding the Cocos plate located in the Pacific Ocean.
As these two chunks of crustal material collide, the rocks of the
Cocos plate are pushed downward, subjecting them to increased heat
and pressure that eventually turns them into molten rock under great
pressure. Because the downward thrust is at an angle to the east,
when the pressure build-up finally becomes too much and the lava and
steam move back upward towards the surface, they do so beneath the
western edge of the Caribbean plate. The result is volcanism.
The earliest volcanoes in the area were submarine ones. As layer after
layer of cooled volcanic material collected, the peaks finally emerged
above the ocean's surface. In this manner, a chain of volcanic islands
formed in a line parallel to and east of the zone of contact between
the two plates.
As the millennia passed and eruptions continued to throw more material
down the slopes of the rising volcanoes, the land area filled in around
their bases. Only as recently as three million years ago was the uninterrupted
land bridge completed between northern Central America and South America,
giving rise to movements of plant and animal species both northward
and southward. This land bridge between two great continents is in
large part responsible for the incredibly high biodiversity found
in an area as small as Costa Rica.
The other prominent factor that explains the country's tremendous
variety of flora and fauna is the range of climatic conditions that
result from the changes in temperature and rainfall as one goes up
and over the mountains.
Costa Rica's climate is influenced by many factors, as is weather
everywhere, although perhaps two of the most important factors are
the fairly even amount of solar radiation received throughout the
year and the prevailing northeasterly winds, known as the trade winds.
Situated at just ten degrees latitude north of the equator, this tropical
nation receives sunlight from a nearly overhead angle year-round and
day length does not vary more than an hour either way from 12-hours
of daylight. This means that annual temperatures remain quite constant
for any particular place in the country at a given hour. In other
words, the temperature in San José, say, at noon averages 25.5º
C in June and 23.5º C in December-hardly a significant difference.
During any 24-hour period there is a somewhat greater range of temperatures
experienced between the daily high and low, although this, too, at
an average of about 8º C, is relatively small compared to many
temperate zone areas.
With more or less constant temperatures found at any given location,
the most important variable in annual climate patterns becomes precipitation.
Rainfall in Costa Rica results from the interaction of the trade winds
with local topography. When moisture-laden air coming in off the Caribbean
Sea encounters the coastline, the difference in surface temperature
between the land and the water can often trigger showers. Moving further
inland the air reaches the eastern foothills of the country's mountainous
backbone. As the air mass rises to pass over the barrier, it cools,
and because cool air can hold less moisture than warm air, it rains,
causing the middle elevations of the Caribbean-facing slopes to be
the wettest areas in the country with average annual precipitation
of more than 4000 mm.
Even though rainfall is fairly evenly distributed throughout the year
on the eastern side of the cordilleras, there is a noticeably drier
period from January through April and a peak in precipitation from
June through August and again in November and December. It's best
to be prepared for rain any day of the year on the Caribbean side
of Costa Rica, unlike the situation that occurs on the other side
of the mountains.
From mid-November through mid-May (on average) the Central Valley
and the northwestern portion of the country are affected by an annual
dry season. The warm moist air driven westward by the trade winds
loses its moisture as it crosses the cordilleras (as described above)
and the resulting dry air gusts down the Pacific slopes drying out
everything in its path. With such low moisture content, few clouds
form to block the sunshine and the prevailing winds keep Pacific breezes
from bringing moisture onshore, thus, further promoting the dryness.
The southern half of the Pacific slope is not normally as strongly
influenced by these effects owing to the fact that the height of the
Talamanca mountain range blocks the drying winds to some degree, which
allows moisture to be brought in from the Pacific Ocean, causing occasional
showers even in the dry season.
As the trade wind belt moves northward in response to global climate
conditions (principally, the angle of the sun and area of greatest
surface heating), Costa Rica enters its rainy season as moist air
flows in from both oceans and convection currents cause showers to
occur. Regional weather conditions, such as tropical waves, tropical
depressions, and even hurricanes farther north and east in the Caribbean,
can greatly affect precipitation levels here. The first two atmospheric
phenomena usually bring increased rainfall to the eastern side of
the country when they pass through the western portion of the Caribbean
Sea. Distant hurricanes (fortunately these major storms almost never
reach Costa Rica -- one hit south of Limón in 1910) can result
in what are known here as temporales del Pacífico. These are
rainy periods lasting two days or more when air from the Pacific,
being drawn in continuously towards the extreme low pressure center
out in the Caribbean, is backed up against the Pacific-facing slopes
of the cordilleras and drops its moisture.
The annual differences in rainfall from one part of the country to
another, together with the change in average temperature from warm
to cool as one moves from sea level up into the mountains, are the
basis for the variety of life zones (tropical dry forest, tropical
wet forest, premontane rain forest, etc.) that exist in Costa Rica,
and also are intimately linked with such biological events as flowering
and fruiting of plants and breeding and migration of animals.