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When talk turns to Costa Rica’s climate, hyperbole flows as thick and as fast as the waterfalls that 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.”
The country lies wholly within the tropics, yet boasts at least a dozen climatic zones and is markedly diverse in local microclimates. Most regions have a rainy season (May–Nov.) and a dry season (Dec.–Apr.). Rainfall almost everywhere follows a predictable schedule. In general, highland ridges are wet, and windward sides always the wettest.
The terms “summer” (verano) and “winter” (invierno) are used by Ticos to designate their dry and wet seasons, respectively. Since the Tican “summer” occurs in what are winter months elsewhere in the Northern Hemisphere (and vice versa), it can be confusing.
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 average 27°C at sea level on the Caribbean coast and 32°C on the Pacific lowlands. In the highlands, the weather is refreshingly clear and invigorating. San José’s daily temperatures are in the low 20s Celsius almost year-round, with little monthly variation, and there’s never a need for air-conditioning. A heat wave is when the mercury reaches the above 27°C. Nights are usually in 16–21°C year-round, so bring a sweater.
Temperatures fall steadily as elevation climbs (about one degree for every 100-meter gain). They rarely exceed a mean of 10°C atop Chirripó (at 3,819 m, the highest mountain), where frost is frequent and enveloping clouds drift dark and ominous among the mountain passes.
Sunrise is around 6 a.m. and sunset about 6 p.m. throughout the year, 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.
Rain is a fact of life in Costa Rica. Annual precipitation averages 250 centimeters nationwide. Depending on the region, the majority of this may fall in relatively few days. The Tempisque Basin in Guanacaste, for example, receives as little as 48 centimeters, mostly in a few torrential downpours. The mountains, by contrast, often exceed 385 centimeters per year, sometimes as much as 7.6 meters on the more exposed easterly facing slopes.
Generally, rains occur in the early afternoons in the highlands, mid-afternoons 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 letup for several days and nights.
Dry season 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 starts in January and runs through April.
Be prepared: 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 in July and August or August and September, particularly along the Pacific coast.
Seasonal patterns can vary, especially in years when the occasional weather phenomenon known as El Niño sets in. . For example, 2008 was the wettest year ever on record—torrential rainfall struck the entire country, causing horrendous flooding and landslides. Rarely do hurricanes strike Costa Rica, although Hurricane César came ashore on July 27, 1996, killing 41 people and trashing the Pacific southwest.
© Christopher P. Baker from Moon Costa Rica, 8th Edition