Costa Rica's seasons are defined by how much rain falls during a particular period and not to the four seasons in the Northern Hemisphere. The year can be split into two periods, the dry season known to the residents as summer, and the rainy season, known locally as winter. The "summer" or dry season goes from December to April, and "winter" or rainy season goes from May to November, which almost coincides with the Atlantic hurricane season, and during this time, it rains constantly in some regions.

The location receiving the most rain is the Caribbean slopes of the Central Cordillera mountains, with an annual rainfall of over 5,000 mm (196.9 in). Humidity is also higher on the Caribbean side than on the Pacific side. The mean annual temperature on the coastal lowlands is around 27 °C (81 °F), 20 °C (68 °F) in the main populated areas of the Central Cordilera, and below 10 °C (50 °F) on the summits of the highest mountains.