The Osa Peninsula and the South Pacific Feature
Understanding Costa Rica's Climate
Although you may associate the tropics with rain, precipitation in Costa Rica varies considerably, depending on where you are and when you're here. This is a result of the mountainous terrain and regional weather patterns. A phenomenon called rain shadow—when one side of a mountain range receives much more than the other—plays an important ecological role in Costa Rica. Four mountain ranges combine to create a continental divide that separates the country into Atlantic and Pacific slopes; because of the trade winds, the Atlantic slope receives much more rain than the Pacific. The trade winds steadily pump moisture-laden clouds southwest over the isthmus, where they encounter warm air or mountains, which make them rise. As the clouds rise, they cool, lose their ability to hold moisture, and eventually dump most of their liquid luggage on the Caribbean side.
During the rainy season—mid-May to December—the role of the trade winds is diminished, as regular storms roll off the Pacific Ocean and soak the western side of the isthmus. Though it rains all over Costa Rica during these months, it often rains more on the Pacific side of the mountains than on the Atlantic. Come December, the trade winds take over again, and while the Caribbean prepares for its wettest time of the year, hardly a drop falls on the western side until May.
Climate variation within the country results in a mosaic of forests. The combination of humidity and temperature helps determine what grows where; but whereas some species have restricted ranges, others seem to thrive just about anywhere. Plants such as strangler figs and bromeliads grow all over Costa Rica, and animals such as the collared peccary and coati—a long-nose cousin of the raccoon—can pretty much live wherever human beings let them. Other species have extremely limited ranges, such as the mangrove hummingbird, restricted to the mangrove forests of the Pacific coast, and the volcano junco, a gray sparrow that lives only around the highest peaks of the Cordillera de Talamanca.
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