Unlike the U.S and other countries, where there are four seasons, Costa Rica has no real summer, fall, spring or winter, only dry and rainy and/or very wet times of the year.
The dry season (or “tourist season”), generally starts during November and last to June.
The wet season starts in June and goes to November and a time when one can get the best deals on hotels and other forms of recreations.
Average temperature is around 75 degrees Fahrenheit, but in the high mountain areas it can get below freezing. Starting in December to April (the peak of the tourist season) normally there is little rain. Keep in mind that Costa Rica is a tropical country, so the average rainfall in the country is around 100 inches, some mountainous areas get over 200 inches of rainfall on a yearly basis with the heaviest rain period between September to October.
The Caribbean coast has its own unique microclimate.
Tradewinds keeps the weather hot and humid most of the year; there is no real dry season and it can rain just about any day or time. Also keep in mine, Costa Rica rarely gets the blunt force of hurricanes, but they do received the aftermath of it, like Tropical Storm Tomas, which hammered the Central Pacific, knocking out road, causing many landslides and even caused a few deaths in November 2010.
Even when Hurricane Ida (2009), Hurricane Gert (1993) and Hurricane Cesar–Douglas (1996) cause sever damaged in other Central America countries, Costa Rica was in reality only side-swiped by them.
Expect the unexpected, just because you plan on a trip and vacation during the dry season, a tropical downpour can happen anytime of the day. Always bring along some weather protection.
If you do get caught in a tropical downpour, our advice, seek shelter in some bar and power down a few Imperials while waiting the few minutes (or a couple of hours) for it to clear up again :-).