Costa Rica is a tropical country and experiences only two seasons: wet and dry. The dry season generally falls between late December and April. This is of course the most popular season to travel, which is why travelers to Costa Rica may find that many others that are visiting the country as well.
Costa Rica's wet season lasts the rest of the year. This time of year can be an invigorating time to travel and experience the country without the interruption of the tourist season. The Caribbean coast is covered by tropical forest and tends to be wetter than other parts of the country year round. That being said, the Caribbean side of the Continental Divide does experience a mild "dry season" period where average rainfalls are slightly lowered during September and October, almost the exact opposite of the Pacific side of the country.
Temperatures vary little between seasons and the main influence on temperature is altitude. San José, at an altitude of 1150m (3772ft) has a climate that feels like spring all year round: lows average 15°C (60°F) and highs average 26°C (79°F). The coastal regions are usually much warmer, with the Caribbean averaging 21°C (70°F) at night and over 30°C (86°F) during the day. The Pacific coast tends to be a few degrees warmer still. The humidity at low altitudes can be quite oppressive.