Costa Rica is a tropical climate. Rain is a near constant, and so is heat with humidity.
While Costa Rica's area is not that big, roughly 300 miles long by 100 miles wide, there are a number of microclimates to pay attention to.
In general, the rainy season extends May through November while the dry season his December through April. These are relative seasons, however, as it is likely to rain almost every day for certain amounts of time. The northeast hillsides of Costa Rica can draw up to 250 inches of rain per year. That is A LOT of precipitation. Most people visit January through March, as a way to beat the northern hemisphere winter.
Temperature variations throughout the year are minimal, with average highs ranging from 77°F during the coolest average month (September) to 85°F during the warmest average month (March). Cooler months in Costa Rica carry a high in the low 70s Fahrenheit while the warmer months carry highs in the low 80s Fahrenheit. Elevation is a key factor in this schedule, and 4° Fahrenheit is the schedule dropped for every thousand feet of elevation gain. Roughly 70 inches of rain falls here as well.
More extreme temperatures can be encountered elsewhere in Costa Rica. Higher temperatures can be found in the Guanacaste region: up to 90°F during the dry season. Torrential rains keep Tortuegero, along the Caribbean coast, wet almost all year long, dropping an average of roughly 115 inches of precipitation along the coast. Over double that falls in the inland mountains.