The weather in Costa Rica is unlike anywhere else in the world. A very small country, about the size of West Virginia, bordered by the Caribbean on the east and the Pacific ocean on the west with tall volcanic mountain ranges in between, creating diverse micro-climates throughout the country.
Most of Costa Rica is defined by two seasons, wet or "green" season and dry or "brown" season. Except for the Caribbean coast, most of the rain falls from the middle of May to the middle of November. December through April see little, if any rain, especially in the more arid Pacific Northwest province of Guanacaste where half of the year is a luscious, almost fluorescent green, and the other half is completely dry. While the hills and valleys will remain green until the beginning of January, by the end of April, all but a few evergreens are parched and mostly brown.
The Pacific Northwest gets about 60 inches of rain a year but nearly half of this falls in September and October. The Caribbean side gets about 125 inches per year but it is spread out more evenly throughout the months. The Central valley falls somewhere in between. When it is the rainiest on the Pacific side during September and October, the Caribbean side has the least amount of rain making this a popular time to visit some of the small Caribbean towns.
View GOES east satellite loop of Costa Rica and Central America
In the beach communities of the Pacific Northwest such as Coco, Playa Flamingo and Tamarindo, the average high is 91 and average low is 71. April and May, when combined with humidity, are the most uncomfortable months. September and October have the most rain but lowest temperatures. It is extremely windy from January through April, sometimes reaching near hurricane force, especially at night. November is gorgeous. The landscape is lush and green, temperatures are only in the mid to high 80s and the rain has subsided. December is a close second. If you like rain, the months of September and October are for you.
San Jose and the surrounding suburbs are at an elevation between 3000 and 3500 feet. Therefore it is much cooler than the Pacific Northwest beach communities. This area also receives a little more rainfall because of the proximity to the western facing mountain slopes that squeeze more rain out of the atmosphere. Many people who live in this area don't require air conditioning.
Limon is located right on the Caribbean and it pretty warm and humid all year long, but still not has hot as the Pacific northwest beach towns. Rainfall is more evenly distributed over the entire year. The Caribbean side has historically been more susceptible to tropical storms and hurricanes as they being for form.