Although Costa Rica is in the tropical belt of the world, and although its temperatures do not vary greatly from one season to the next, they do vary from one area to the next. For example, San Gerardo de Dota has an average temperature of 62oF (16oC), while the coast is always warm enough for a tank top (82oF or 28oC).
Precipitation, on the other hand, does vary greatly from season to season. There will be much more rain between May and November than between December and April, although there is always a chance of rain in the tropics and if you go to places like the cloud forest in Monteverde, you’ll be best prepared with a rain jacket. When you plan your trip to Costa Rica, it’s best to be on the safe side and pack at least one set of warm clothes, a rain jacket and then all the summer clothes you want.