Climate: Costa Rica's weather 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 weather 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
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 climatic 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