The Costa Rica climate is more varied than you might think. Altitude and location create lots of micro-climates, and you'll want to know which one suits you best before you decide where to retire in Costa Rica.
Tim Lytle, on his excellent web site TheRealCostaRica.com says:
Costa Rica has so many climates that a visitor can be surprised by the need for change of clothing.
In the mountains, night time temperatures can be as low as 10°C (50°F), whereas the average night along the Caribbean coast feels with 21°C (70°F) like a warm day in Ireland! Average day time temperatures in the coastal low lands are over 30°C (86°F).
climate all year round is found in San Jose and the surrounding
so-called Central Valley. The Central Valley or Meseta
Central is not really a valley, but rather a plateau with
altitudes between 900m and 1500m (3000 to 5000 feet). With average
temperatures between 14 to 27°C (57 to 81°F), you won't need heating
Costa Rica has - like many tropical countries - a distinct dry and rainy season. During the dry season, also called summer, from December to early May it rarely rains. Hotels and tours are fully booked, beaches crowded and the kids enjoy their summer holidays from December to February.
The rainy season, also referred to as winter or "green season", lasts from late May to late November. During the green season, the weather in Costa Rica goes like this: you'll wake up to a sunny morning, you watch some clouds build up over noon / afternoon, you get drenched in a good tropical downpour (if you are outside, that is). So, even during the rainy season, you'll get plenty of sun shine.
The average rainfall increases along the Pacific coast from north to south. Guanacaste, often called the "Gold Coast", receives about 140cm (55 inches) per year and the most hours of sun shine - hence the name. In Puntarenas along the South Pacific coast it can rain up to 500cm (200 inches) per year.
Caribbean coast is said to be hot and humid all year long. However, if
you look at the Costa Rica rainfall map at Toucan
Guides, you'll notice that some areas along the Caribbean coast receive
only 150 to 200cm (60 to 80 inches) of rain per year - not much more
than the Guanacaste region.