The region is a crucible. There are few places in the world where the forces of nature so actively interplay. Distinct climatic patterns clash and merge; the great landmasses and their offshore cousins, the Cocos and Caribbean plates, jostle and shove one another, triggering earthquakes and spawning sometimes cataclysmic volcanic eruptions; and the flora and fauna of the North and South American realms--as well as those of the Caribbean and the Pacific--come together and play Russian roulette with the forces of evolution. The result is an incredible diversity of terrain, biota, and weather concentrated in a country barely bigger than the state of New Hampshire.

At 50,895 square kilometers, Costa Rica is the second-smallest Central American nation after El Salvador. At its narrowest point, in the south, only 119 km separate the Caribbean from the Pacific. Even in the north one can savor a leisurely breakfast on the Caribbean and take an ambling five-hour drive to the Pacific for dinner. At its broadest point, Costa Rica is a mere 280 km wide. On the ruler-straight eastern seaboard, barely 160 km separate the Nicaraguan and Panamanian borders. And while the Pacific coast is longer, it is still only 480 km from the northernmost tip to the Panamanian border as the crow flies.

Lying between 8 and 11 degrees north of the equator, Costa Rica sits wholly within the tropics, a fact quickly confirmed in the middle of a rainy afternoon in the middle of the rainy season in the middle of the sodden Caribbean lowlands or the Talamanca mountains. Elevation and extremes of relief, however, temper the stereotypical tropical climate. In fact, the nation boasts more than a dozen distinct climatic zones. Atop the highest mountains in cooler months, even ice and snow aren't unknown.

A Backbone of Mountains
Costa Rica sits astride a jagged series of volcanoes and mountains, part of the great Andean-Sierra Madre chain, which runs the length of the western littoral of the Americas. From the Pacific coast of Costa Rica, great cones and domes dominate the landscape, and you're usually in sight of volcanoes in the northern part of the country.

The mountains rise in the nation's northwestern corner as a low, narrow band of hills. They grow steeper and broader and ever more rugged until they gird Costa Rica coast to coast at the Panamanian border, where they separate the Caribbean and Pacific from one another as surely as if these were the towering Himalayas.

Volcanic activity has fractured this mountainous backbone into distinct cordilleras. In the northwest, the Cordillera de Guanacaste rises in a leapfrogging series of volcanoes, including Rincón de la Vieja and Miravalles, whose steaming vents have been harnessed to provide geothermal energy. To the southeast is the Cordillera de Tilarán, dominated by Arenal, one of the world's most active volcanoes. To the east and rising even higher is the Cordillera Central, with four great volcanoes--Poás, Barva, Irazú and Turrialba--that gird the central highlands and within whose cusp lies the Meseta Central, an elevated plateau ranging in height from 900 to 1,787 meters. To the south of the valley rises the Cordillera Talamanca, an uplifted mountain region that tops out at the summit of Cerro Chirripó (3,819 meters), Costa Rica's highest peak.

Meseta Central
In Costa Rica, all roads radiate from the Meseta Central, the heart and heartbeat of the nation. This rich agricultural valley is cradled by the flanks of the Cordillera Talamanca to the south, and by the fickle volcanoes of the Cordillera Central to the north and east. San José, the capital, lies at its center. At an elevation of 1,150 meters, San José enjoys year-round temperatures above 21° C (70° F), reliable rainfall, and rich volcanic soils--major reasons why almost two-thirds of the nation's population lives in the valley.

The Meseta Central--which measures about 40 km north to south and 80 km east to west--is divided into two separate valleys by the low-lying crests of the Cerros de la Carpintera, which rise a few miles east of San José. Beyond lies the somewhat smaller Cartago Valley, at a slightly higher elevation. The Carpinteras mark the Continental Divide. To the east the turbulent Reventazón--a favorite of white-water enthusiasts--tumbles helter-skelter to the Caribbean lowlands. The Río Virilla exits more leisurely, draining the San José Valley to the west.

Northern Lowlands and Caribbean Coast
The broad, pancake-flat, wedge-shaped northern lowlands are cut off from the more densely populated highlands by a languorous drape of virtually impenetrable hardwood forest that only the most accomplished outdoor types can penetrate without local guides. The low-lying plains or llanuras, which make up one-fifth of the nation's land area, extend along the entire length of the Río San Juan, whose course demarcates the Nicaraguan border. Farther south the plains narrow to a funnel along the Caribbean coast.

Westward, cattle ranches and banana and citrus plantations give way to pleats of green velveteen jungle ascending the steep eastern slopes of the central mountains, which run along a northwest-southeast axis, forming the third side of the wedge. Numerous rivers drop quickly from the mountains to the plains, where they snake along sluggishly. Beautiful beaches, many of gray or black sand, line the Caribbean coast, which sidles gently south.

Pacific Coast
Beaches are a major calling card of Costa Rica's Pacific coast, which is deeply indented with bays and inlets and two large gulfs--the Golfo de Nicoya (in the north) and Golfo Dulce (in the south), enfolded by the hilly, hook-nosed peninsulas of Nicoya and Osa, respectively. Mountains tilt precipitously toward the Pacific, coming closer to the ocean here than on the Caribbean side, and the slender coastal plain is only a few kilometers wide. North of the Golfo de Nicoya, the coastal strip widens to form a broad lowland belt of savanna--the Tempisque Basin. The basin is drained by the Río Tempisque, and narrows northward until hemmed in near the Nicaraguan border by the juncture of the Cordillera de Guanacaste and rolling, often steep, coastal hills that follow the arc of the Nicoya Peninsula.

Of growing importance to the national economy is the narrow, 64-km-long intermontane basin known as the Valle de El General, which runs parallel to and nestles comfortably between the Cordillera Talamanca and the coastal mountains--Fila Costeña--of the Pacific southwest. The Ríos General and Coto Brus and their many tributaries have carved a deep, steep-sided trough, long isolated from the rest of the nation, although the construction of the Pan-American Highway through the valley in the 1950s brought thousands of migrant farmers and their families in its wake.

Costa Rica lies at the boundary where the Pacific's Cocos Plate--a piece of the earth's crust some 510 km wide--meets the crustal plate underlying the Caribbean. The two are converging as the Cocos Plate moves east at a rate of about 10 cm a year. It is a classic subduction zone in which the Caribbean Plate is forced under the Cocos--one of the most dynamic junctures on earth. Central America has been an isthmus, a peninsula, and even an archipelago in the not-so-distant geological past. It has therefore been both a corridor for and a barrier to landward movements, and it has been an area in which migrants have flourished, new life forms have emerged, and new ways of life have evolved. Yet a semblance of the Central America we know today became recognizable only in recent geological history. In fact, Costa Rica has one of the youngest surface areas in the Americas--only three million years old--for the volatile region has only recently been thrust from beneath the sea.

Earthquakes
In its travels eastward, the Cocos Plate gradually broke into seven fragments, which today move forward at varying depths and angles. This fracturing and competitive movement causes the frequent earthquakes with which Costa Ricans contend. The forces that thrust the Cocos and Caribbean Plates together continue to build inexorably.

From insignificant tremors to catastrophic blockbusters, most earthquakes are caused by the slippage of masses of rock along earth fractures or faults. Rocks possess elastic properties, and in time this elasticity allows rocks to accumulate strain energy as tectonic plates or their component sections jostle each other. Friction can contain the strain and hold the rocks in place for years. But eventually, as with a rubber band stretched beyond its breaking point, strain overcomes frictional lock and the fault ruptures at its weakest point.

Suddenly, the pent-up energy is released in the form of an earthquake--seismic waves that radiate in all directions from the point of rupture, the "focus." This seismic activity can last for a fraction of a second to, for a major earthquake, several minutes. Pressure waves traveling at five miles per second race from the quake's epicenter through the bedrock, compressing and extending the ground like an accordion. Following in their wake come waves that thrust the earth up and down, whipping along at three miles per second.

For Costa Ricans, the bad news is that the most devastating earthquakes generally occur in subduction zones, when one tectonic plate plunges beneath another. Ocean trench quakes off the coast of Costa Rica have been recorded at 8.9 on the Richter scale and are among history's most awesome, heaving the sea floor sometimes scores of feet. These ruptures often propagate upward, touching off other, lower-magnitude tremors. This is what happened when the powerful 7.4 quake struck Costa Rica on 22 April 1991. That massive quake, which originated near the Caribbean town of Pandora (112 km southeast of San José), left at least 27 people dead, more than 400 injured, 13,000 homeless, and more than 3,260 buildings destroyed in Limón Province. The earthquake caused the Atlantic coastline to rise permanently--in parts by as much as 1.5 meters. In consequence, many of the beaches are deeper, and coral reefs have been thrust above the ocean surface and reduced to bleached calcareous skeletons.

Volcanoes
Costa Rica lies at the heart of one of the most active volcanic regions on earth. The beauty of the Costa Rican landscape has been enhanced by volcanic cones--part of the Pacific Rim of Fire--that march the length of Central America. Costa Rica is home to seven of the isthmus' 42 active volcanoes, plus 60 dormant or extinct ones. Some have the look classically associated with volcanoes--a graceful, symmetrical cone rising to a single crater. Others are sprawling, weathered mountains whose once-noble summits have collapsed into huge depressions called calderas (from the Portuguese word for "cauldron"). Still others, such as on Cocos Island, have smooth, shield-shaped outlines with rounded tops pocked by tiny craters.

Visitors seeking to peer into the bowels of a rumbling volcano can do so easily. The reward is a scene of awful grandeur. Atop Poás's crater rim, for example, you can gape down into the great well-like vent and see pools of molten lava bubbling menacingly, giving off diabolical, gut-wrenching fumes of chlorine and sulfur--and, for good effect, emitting explosive cracks, like the sound of distant artillery.

Several national parks have been created around active volcanoes, with accommodations, viewing facilities, lectures, and guided walks to assist visitors in understanding the processes at work. A descriptive map charting the volcanoes is published by the Vulcanological and Seismological Observatory of Costa Rica, at the National University in Heredia, which monitors volcanic activity throughout the nation. An excellent descriptive guide is Guillermo Alvarado's Costa Rica; Land of Volcanoes, which offers scientific explanations in layperson's terms.

In 1963, Irazú (3,412 meters) broke a 20-year silence, disgorging great clouds of smoke and ash. The eruptions triggered a bizarre storm that showered San José with 13 cm of muddy ash, snuffing out the 1964 coffee crop but enriching the Meseta Central for years to come. The binge lasted for two years, then abruptly ceased (although it began rumbling again in 1996). Poás (2,692 meters) has been particularly violent during the past 30 years. In the 1950s, the restless four-mile-wide giant awoke with a roar after a 60-year snooze, and it has been huffing and puffing ever since. Eruptions then kicked up a new cone about 100 meters tall. Two of Poás's craters now slumber under blankets of vegetation (one even cradles a lake), but the third crater belches and bubbles persistently. Volcanologists monitor the volcano constantly for impending eruptions.

Arenal (1,624 meters) gives a more spectacular light and sound show. After a four-century-long Rip van Winkle-like dormancy, this 4,000-year-young juvenile began spouting in 1968, when it laid waste to a four-square-mile area. Arenal's activity, sometimes minor and sometimes not, continues unabated; it erupted spectacularly in August 2000, killing a Tico tour guide and forcing the evacuation of Tabacón. Though more placid, Miravalles, Turrialba, and Rincón de la Vieja, among Costa Rica's coterie of coquettish volcanoes, also occasionally fling fiery fountains of lava and breccia into the air. Rincón blew in 1995, doing damage in Upala.

The type of magma that fuels most Central American volcanoes is thick, viscous, and so filled with gases that the erupting magma often blasts violently into the air. If it erupts in great quantity, it may leave a void within the volcano's interior, into which the top of the mountain crumbles to form a caldera. Irazú is a classic example. Its top fell in eons ago. Since then, however, small eruptions have built up three new volcanic cones--"like a set of nesting cups," says one writer--within the ancient caldera.

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." 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 a 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-November) and a dry season (December-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--occurs in what are winter months elsewhere in the Northern Hemisphere (and vice versa) it can be confusing. Don't be put off by the term "rainy season." Costa Rica promotes it as the "green season"--and it's a splendid time to travel, generally.

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 average 27° C (82° F) at sea level on the Caribbean coast and 32° C (89° F) on the Pacific lowlands. Balmy San José and the Meseta Central have an average year-round temperature of 23° C (74° F). Temperatures fall steadily with elevation (about one degree for every 100-meter gain). They rarely exceed a mean of 10° C (48° F) atop Chirripó, where frost is frequent and enveloping clouds drift dark and ominous 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.

The length of daylight varies only slightly throughout the year. Sunrise is around 6 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 250 cm (100 inches) nationwide. Depending on the region, the majority of this may fall in relatively few days--sometimes fewer than 15 per year. In drier years, the Tempisque Basin in Guanacaste, for example, receives as little as 48 cm (18 inches), mostly in a few torrential downpours. The mountains, by contrast, often exceed 385 cm (150 inches) per year, sometimes as much as 7.6 meters (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, 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 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 occurs January-April.

Even in the rainy season, days often start out warm and sunny, although temporales (morning rainfall) are not uncommon. As in many tropical destinations worldwide, only newly arrived gringos go out without an umbrella after noon during the wet season. In the highlands, rainy season usually brings an hour or two of rain midafternoon. Still, 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 July-August or August-September, particularly along the Pacific coast.

Be aware, though, that seasonal patterns can vary, especially in years when the occasional weather phenomenon known as El Niño may set in, as it did with devastating force in 1997. The freak weather it produces is caused by an abnormal warming of ocean waters off the Pacific coast, most often when warm currents from the Western Pacific shift, resulting in volatile changes in air masses.

Rarely do hurricanes strike Costa Rica, although Hurricane Cesár came ashore on 27 July 1996, killing 41 people and trashing the Pacific southwest in the nation's worst national disaster in a decade. Large-scale deforestation in the region contributed to massive flooding.

When To Go
Although the best time weather-wise is December-May, Costa Rica is a year-round destination. Most of my research for this book was undertaken in the wet (green) season. During Christmas week and Easter, the whole of Costa Rica seems to descend on the beach. Otherwise, any time is a good time (but do take note of regional variations in weather).