|Costa Rica lies wholly within the tropics yet boasts at least one dozen climatic zones and is markedly diverse in microclimates, which make generalizations and rainfall misleading.Most regions on Costa Rica have two different seasons, the rainy season from May to November and the dry season from December to April. And the rainfall almost everywhere follows a predictable schedule. In general, high ridges are wet, and windward sides always the wettest.Temperatures are more dictated by location and elevation than by season, range from tropical on the coastal to temperate in the interior highlands. Mean temperatures hover near 22°C (72°F) on the central plateau, average 27°C (82°F) at sea level on the Caribbean coast and 32°C (89°F) and the pacific lowlands, and fall steadily with elevation. Balmy San Jose and the Meseta Central have an average year-round temperature of 23°C (74°F). Everywhere in Costa Rica from March to May are the hottest months, with October and September not so far behind.
Generally, rains occur in the early afternoons in the highlands, midafternoons in the pacific lowlands, and late afternoons and night in the Atlantic lowlands. 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 north west coast which is 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 from January to April.
Even in the rainy season, days often start out warm and sunny, although temporales are not uncommon. In the highlands, rainy seasons usually means an hour or two of rain in the midafternoon. Still be prepared because 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, sometimes occurs during July-August or August-September particularly along the pacific coast.
Courtesy of http://www.infocostarica.com