When to go?

Costa Rica Climate
Being located in the tropical region, the climate of Costa Rica is not ruled by the same mechanisms as the climate in higher latitudes.  We don’t have the four seasons and the days are very close to 12 hours of light in any time of the year.
Therefore the answer is:  you can come any time!
 
Temperatures do not change much with the time of year either, but in the drier months, when the sky is clear and the sun is shining, they definitely are higher than when there are clouds in the sky.  Altitude does make a big difference in the temperatures.
 
The coastal areas of the Pacific and the flat lands of the northeast average above 25 C (77 F.). Some popular towns in these regions are Tamarindo, Papagayo, Jacó, Manuel Antonio and Tortuguero.
 
Further inland at the base of the mountains and on the southern Caribbean coast, the average is between 20 C and 26 C (68 F to 78 F).  In this range we find La Fortuna, Sarapiquí, San Isidro de Pérez Zeledón and Puerto Viejo de Limón.
 
The Central Valley, located at 1100 mts of altitude, and all the mid elevation regions are considered to have the best temperatures in the world.  We call it the “ethernal spring”.  The annual average is 12 C to 20 C (54 F to 58 F)
 
Our mountain tops including Poás and Irazú Volcanoes, Cerro Chirripó and Cerro de la Muerte, can get pretty cold with averages below 12 C (54 F)
 
For such a small country, the contrasts in weather patterns between the different parts of the country are dramatic.  We’ll over simplify here and talk about four micro climate regions.
 
The Caribbean coast and the northern flatlands
These regions are exposed to the trade winds blowing year round from the northeast.  These winds bring humid air from the Caribbean which causes precipitation on the flatlands and at the base of the mountains.  It can rain any day of the year with a short dry spells in March-April and September
 
The closer you are to the coast the more it rains: Tortuguero and Limón are very rainy.  Regions further west get substantially less rain:  La Fortuna, Upala for example.
 
The higher you are on the mountain slope, the more it rains also:  the Orosi Valley and the Reventazón and Pacuare start here.
 
The Pacific Coast
During most of the year our Pacific coast gets warm humid winds from the ocean causing the afternoon rains, so common in most tropical areas.   These rains, lasting between one and three or four hours, are very regular between mid May and mid November except for a short period of about two weeks in July that we call “veranillo”.  Most of the time the evenings are again clear and nice.
 
On the opposite time of the year, the presence of the winter in the northern hemisphere causes low air pressure over the Pacific Ocean, and from December to April our Pacific coast dries out.  The trade winds, which keep blowing all this time from the Caribbean, deposit all their moisture as they travel across the country.  By the time they reach the top of the mountains and make their way over to the Pacific slope, they are totally dry and the regions of Guanacaste, Puntarenas and as far south as Jacó, enjoy perfectly clear skies day after day.
 
Rain in Osa Peninsula
The southern Pacific coast, including Manuel Antonio, Dominical and the Osa Peninsula, get considerably more rain than the rest of the country, except maybe some regions in the east coast.  They still have a short dry period lasting from December to February.
 
The Central Valley
The Central Valley has a weather pattern very similar to the northern Pacific coast.  The mountains protect it from the humid trade winds and allow a nice dry season from December to May.  Even better here, the temperatures are a lot lower because of the altitude.