Temperature

Temperatures  don't change much throughout the year. Generally speaking, the difference in the mean temperature of the warmest and coolest months is no more than five degrees Cº   (Celsius).

Temperatures vary in Costa Rica mostly, but no uniquely, because of Altitude. Below 800 meters the temperature can be between  25 to 32°C, in regions of altitude from  800 to 2,500 the temperatures vary from 14 and 25°C like in the central valley , and above 2,500 the temperatures are below 14°C.

Rainfall

Costa Rica has two main rainfall regions,  one on the Caribbean side and the other on the Pacific side, this includes the northern, central, and southern regions, as well as the Central Plateau.

As mentioned before, the main change in seasons is the amount of rainfall. During the rainy season, that extends  from May to the middle of November in the northern regions, and from April to December in the south. The dry season takes in the rest of the year, and it's defines by two phases, first (December, January, February)  cool  cloudy skies and chilly winds, then (March and April) the sky remains clear for most of the day, with light breezes, and the highest temperatures of the year are reached. The light breezes and absence of rain favors the formation of fog due to the accumulation of dust and pollutants in air. During the dry season, the differences between the maximum, day temperatures and minimum at night  are  pronounced. The rainiest month  in the north, and in the Central Tectonic Depression is September, while it's October in the south. 

Rains usually occur in the afternoon or evening, so if you plan to travel to Costa Rica keep this in mind and enjoy the sun during the morning. During July and August there are short dry spells called "veranillos" ("little summers"). On the Caribbean side, including the northern and Caribbean regions, there is no definite dry season.  In the coastal zone there are, however, relatively dry periods, one in March and April and the other in September and October.  Here the rain usually occurs late in the afternoon or at night.

Winds

The trade winds blowing from the east to the northeast prevail in Costa Rica throughout the year.  Usually, their velocity is below 15 km/hour, but in January and February they may reach 30 km/hour.  On the Pacific side breezes come out of the west, from April to October, favoring the formation of rain clouds.  These breezes are light on velocity, around 10 km/hour.