Seasons vary throughout the year depending on where you are in the country. So, in order to create a tool that can help a better understanding we have divided the country in four climate areas that will help you clarify this.
North Area and Caribbean Coast Area:
Late February, March and April. September and October.
North Caribbean: Tortuguero.
South Caribbean: Cahuita, Puerto Viejo, Manzanillo, and Bocas del Toro (Panama). This may include Turrialba Volcano area.
North Area: Sarapiquí and Arenal Volcano and some áreas in the Central Volcanic Mountain Range (Poas Volcano, Waterfall Gardens).
Sunny Season:. Late December (Around the 20th.) to late May (Around the 20th ).
December and January are usually chillier and windy. (Temperature comes down a couple of degrees in these two months). There are two short periods (Or two weeks each) in late June and Early November when it is sunny.
Central Valley: San José, Alajuela, Heredia, Cartago and San Ramón.
Talamanca Mountain Range: Zona de los Santos (San Gerardo de Dota).
North and Central Pacific Coast:
Sunny Season: Early November to late May.
January can be very windy in Monteverde, Miravalles and Rincon de la Vieja.
North Pacific Coast: Guanacaste
North Pacific Mountains: Rincón de la Vieja, Monteverde.
Central Pacific Area: Jacó, Manuel Antonio and Dominical
South Pacific Area:
Sunny Season: December to June.
South Pacific Area: Osa Peninsula, Golfo Dulce.
This calendar may seem a bit complicated but it is important to realize that in the tropics there are no weather fixed rules. January and February, can be influenced by North cold fronts (And every now and then even March). September and October by hurricanes, and December by the trade winds… Always recommend rain gear! No matter what!