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When talk turns to Costa Rica's climate, hyperbole flows as thick and as fast as the waterfalls which cascade in ribbons of quicksilver down through the forest-clad mountains. English 19th-century novelist Anthony Trollope was among the first to wax lyrical: "No climate can, I imagine, be more favorable to fertility and to man's comfort at the same time than that of the interior of Costa Rica." Merlin the wizard couldn't have conjured the elements into a more blissful climate.
The country lies wholly within the tropics yet boasts at least one dozen climatic zones and is markedly diverse in local microclimates, which make generalizations on temperature and rainfall misleading.
Most regions have a rainy season (May-Nov.) and a dry season (Dec.-April). And the rainfall almost everywhere follows a predictable schedule. In general, highland ridges are wet, and windward sides always the wettest.
When planning your trip, don't be misled by the terms "summer" and "winter," which Ticans use to designate their dry and wet seasons. Since the Tican "summer"--which in broad terms lasts December through April--equates to winter months elsewhere in the Northern Hemisphere, and vice versa, it can be confusing.
Temperatures
Temperatures, dictated more by elevation and location than by season, range from tropical on the coastal plains to temperate in the interior highlands. Mean temperatures hover near 72deg. F on the central plateau, average 82deg. F at sea level on the Atlantic coast and 89deg. F on the Pacific lowlands, and fall steadily with elevation (about one degree for every 100-meter gain). They rarely exceed a mean of 48deg. F atop Chirripó, where frost is frequent and enveloping clouds drift dark and ominously among the mountain passes. You'll definitely need a warm sweater or jacket for the mountains, where the difference between daytime highs and nighttime lows is greatest. Balmy San José and the Meseta Central have an average year-round temperature of 74deg. F.
This being the tropics, the length of daylight varies only slightly throughout the year. Sunrise is around 5 a.m. and sunset about 6 p.m., and the sun's path is never far from overhead, so seasonal variations in temperatures rarely exceed five degrees in any given location.
Everywhere, March to May are the hottest months, with September and October not far behind. Cool winds bearing down from northern latitudes lower temperatures during December, January, and February, particularly on the northern Pacific coast, where certain days during summer (dry season) months can be surprisingly cool. The most extreme daily fluctuations occur during the dry season, when clear skies at night allow maximum heat loss through radiation. In the wet season, nights are generally warmer, as the heat built up during the day is trapped by clouds.
Rainfall
Rain is a fact of life in Costa Rica. The winds and weather of two great oceans
meet above Costa Rica's jungles and mountains. Oceans--especially in tropical
latitudes--spell moisture, and mountains spell condensation. Annual
precipitation averages 100 inches nationwide.
Depending on the region, the
majority of this may fall in relatively few days; sometimes less than 15 days a
year. The Tempisque Basin in Guanacaste, for example, receives as little as 18
inches in drier years, mostly in a few torrential downpours. The mountains, by
contrast, often exceed 150 inches per year, sometimes as much as 25 feet on the
more exposed easterly facing slopes! And don't expect to stay dry in the
montane rainforests even on the sunniest days, for the humid forests produce
their own internal rain as water vapor condenses on the cool leaves and falls.
Generally, rains occur in the early afternoons in the highlands, midafternoons in the Pacific lowlands, and late afternoons (and commonly during the night) in the Atlantic lowlands. Sometimes it falls in sudden torrents called aguaceros, sometimes it falls hard and steady, and sometimes it sheets down without let up for several days and nights. Sounds like England, doesn't it!
Dry season--"summer"--on the Meseta Central and throughout the western regions is December through April. In Guanacaste, the dry season usually lingers slightly longer; the northwest coast (the driest part of the country) often has few rainy days even during wet season. On the Atlantic coast, the so-called dry season occurs January-April.
Even in the rainy season, days often start out warm and sunny, although temporales (morning rainfall) are not uncommon. Like many tropical destinations worldwide, only newly arrived gringos go out without an umbrella after noon during the wet season. Be prepared: in the rainy season, 23 hours of a given day may be dry and pleasant; during the 24th, the rain can come down with the force of a waterfall. The sudden onset of a relatively dry period, called veranillo (little summer), sometimes occurs during July-August or August-September, particularly along the Pacific coast.