The country is divided by a backbone of volcanoes and mountains, an extension of the Andes-Sierra Madre chain which runs along the western side of the Americas. Costa Rica has four distinct cordilleras or mountain ranges -- Guanacaste and Tilaran in the north, Central and Talamanca in the south. Costa Rica is part of the Pacific "Rim of Fire" and has seven of the isthmus's 42 active volcanoes plus dozens of dormant or extinct cones. Earth tremors and small quakes shake the country from time to time. The last major quake hit on April 22, 1991. Centered on the Caribbean side southeast of San Jose, it measured 7.4 on the Richter scale. The country's highest point is Mt. Chirripo (3,797 meters). The capital, San Jose, and the neighboring major cities of Alajuela and Heredie lies in the middle of the Meseta Central (Central Valley). Almost two-thirds of the nation's population live in this small, fertile valley. The Pacific coastal plain is much narrower than its Caribbean counterpart. Both coasts are lined with white and black sand beaches. Costa Rica is a tropical country which contains several distinct climatic zones. There is no winter or summer as such and most regions have a rainy season from May to November and a dry season from December to April. Annual rainfall averages 100 inches nationwide with some mountainous regions getting as much as 25 feet on exposed eastern slopes. Temperature is more a matter of elevation than location with a mean of around 72 degrees in the Central Valley, 82 degrees on the Atlantic coast and 89 degrees on the Pacific coast.