Costa Rica is a tropical country, so with the exception of the cloudforest temperatures range from 25°c – 35°c all year round.
Costa Rica has two seasons: rainy and dry. The dry season runs from December to May. The rainy season is heaviest in September and October so we advise against travel in these months.
The general rule is the further north you are, the drier it will be so Rincon de la Vieja is the driest and hottest part of the country.
The rainy season is a good time to travel to avoid the crowds. Rains usually last only a couple of hours and do not cause any problems for visitors.
Having said this, you can expect some showers throughout the year in the Caribbean wetlands of Tortuguero and the rainforests of Pacuare and the Osa Peninsula.
The cloudforest regions of Monteverde, Bosque de Paz and San Gerardo are at fairly high altitude, so can feel chilly throughout the year, especially at night. It is also wet in the cloudforest so prepare for cool and damp conditions when packing.
The Pacific coast generally is warm and pleasant. In the rainy season you can expect a short downpour late each afternoon so conditions are still pleasant.
Again, in September and October rains are generally much more sustained in the Central and Pacific regions.
Just to confuse things, the South Caribbean coast around Puerto Viejo experiences the opposite seasonal rainfall to the rest of the country. Here, August to October are generally the driest months.