Given Costa Rica's latitude--between 8 and 12 degrees north of the equator--day length and temperature do not change drastically with the seasons. The sun rises around 5 a.m. and sets around 6 p.m. year-round. Temperature differences are found by changing altitude:
- The misty highlands are in the 10-13 degree C (50 to 55F) range.
- The Central Valley, at 3800 feet, averages 26 degrees C (78F).
- At sea level, the temperature is 30-35 degrees C (85-95oF), tempered by sea breezes on the coast.
Slight variations occur in December, January, and February, due to cold winds from the North American winter. These cooler temperatures bring on the dry season or "summer," as Central Americans call it, which lasts from December through April.
Temperatures start to rise as the sun approaches a perpendicular position over Costa Rica. This causes increased evaporation and brings on the rainy season, or "winter," which lasts from April through November, except for a two-week dry season, a time called el veranillo de San Juan (the "little summer"), which occurs sometime in June or July.
Costa Rica's weather pattern is changing and is not as predictable as it used to be. Now there are many dry days during the "winter" and a few storms during the "summer." Here's a new rule of thumb: the more gloriously sun-drenched the morning during the rainy season, the harder it will rain in the afternoon. Conversely, on a cloudy morning there will be less evaporation, and thus a generally drier day.
The Atlantic CoastThe Atlantic Coast has always been an exception to the rule. Trade winds laden with moisture from the Caribbean approach Costa Rica from the northeast. As the moisture rises to the chilly heights of the Cordillera, it condenses into rain on the eastern slopes. For this reason, there is no definite dry season in the Atlantic zone, but the beaches tend to be sunnier than the mountains.
Residents insist that the rainiest months in the rest of Costa Rica, September and October, are the driest on the Caribbean coast. Often you can see Arenal volcano best in September and October, because it tends to share the Atlantic weather pattern. In a similar phenomenon, trade winds from the southeast discharge their moisture against the mountains that separate the Osa Peninsula from the rest of the country. The Atlantic plains and the Osa both receive 150 to 300 inches of rain a year, compared to an average of 100 inches in the Central Valley.
The Central ValleyOne of the most surprising things for newcomers to the Central Valley is that it's not as warm as they expected. December, January, and February are usually rain-free, but the weather can be downright chilly, especially at night or if a wind is blowing.
During the rainy season, May to November, the days tend to start out warm and sunny but cloud over by noon. The downpour usually starts around 2 or 3 p.m. and it can get pretty cold then, too. Usually a sweater and long pants are enough to keep you warm.
When it rains, it really rains, but afternoon downpours are usually short-lived. If you go down in altitude from San José's 3800 feet, you'll be able to wear the kind of clothes you hoped you could wear in the tropics.