Costa Rica is a beautiful country, which will charm you at any time of year. However, many travelers may prefer to take advantage of Costa Rica's dry season, while others will want to take advantage of the off-season deals and less-crowds during the rainy season.
Costa Rica's temperature is steady throughout, averaging between 71 and 81 degrees Fahrenheit. Higher elevations are cooler than lower parts of Costa Rica. Mid-level elevations are usually 5 or 10 degrees cooler, while high elevations, such as Volcanos Poas, Irazu and Chirripo (the second highest mountain in Central America) can dip into the 50s.
In general, Costa Rica's tropical climate is driest between December and April. In some northwestern parts of Costa Rica, such as Guanacaste and Nicoya, the air dries out completely and the climate becomes hot and dry, to the point that sustaining agriculture requires irrigation.
Beaches in those regions are very popular during the dry season, and can get crowded throughout December and popular school holidays like spring break.
On the Caribbean side of the mountains, one thing to watch out for in January and early February is the temporales del Atlantico. When polar air pushes south across the Gulf of Mexico, it picks up a lot of water. This can result in several continuous days of heavy rain in the northeast.
Costa Rica's rainy season falls between May and November, with the heaviest rain falling in October. Throughout that month, the rain is unrelenting. Fortunately, even during the rainy season, it doesn't rain all the time. Up until late June, it's usually mostly sunny. The rest of the time, most days will be sunny and very humid, with a couple of hours of afternoon rain.
The heaviest rain usually falls to the northeast and southwest of Costa Rica. On the Pacific coast, so much rain falls on the Osa Peninsula and Drake Bay during September and October that some lodges temporarily close.
Escaping the rain can be as simple as crossing the country. In September and October, while the west coast of Costa Rica is drenched with rain, the east Caribbean side is often sunny and dry. When the wind goes the opposite way in January and February and brings the polar air down along the Caribbean coast, the opposite is true.
All that wet weather is usually confined to one side of Costa Rica by the spine of mountains running down the middle. The Tilaran and Cordillera mountain ranges are part of the Continental Divide. When the storms start coming in off the Pacific, the mountain ranges keep most of the precipitation just to the west. When heavy tropical storms come in from the Caribbean Sea, the mountains keep the rain from crossing the country.
However, if you plan on exploring Costa Rica's world-famous rain forest, you are going to get wet, no matter what time of year you go. Rain forests thrive on rain. The higher you go in the mountains, the wetter it will be. It may be slightly drier if you visit between December and April, but it won't be dry.
High in the Tilaran Range, it is so wet that the Monteverde nature preserve gets all its water directly from the clouds. In this cloud forest, the moisture from the clouds constantly condenses onto the leaves above you, so your choices range between dripping and soaked. In the wet season, cloud forests get about 10% more rainfall than lower altitude rain forests. In the dry season, the amount of rainfall can be doubled!