The Climate and Enviornment of Costa Rica

Costa Rica is unequivocally a tropical country, situated fairly close to the equator. Although in the mountains above 2000 meters you get much cooler temperatures, the average annual temperature for most of the country lies between 21.7°C (71°F) and 27°C (81°F). The coolest months are from November through January, and the warmest from March through May. San José, the capital, where over a third of the population lives, stands at approximately 1170 meters altitude and averages an annual temperature of 20.6°C (69°F).

The nation's climate is classically divided into two major seasons: rainy and dry. The dry season runs from January through May and the rainy season from May to November and sometimes December.

Weather in the tropics is essentially a phenomenon of solar radiation and air circulation. Intense heat at the equator puts air in motion, and a worldwide pattern of winds is established. The most famous of these, for Costa Rica, are the north-easterly trade winds, known locally as "alisios." These winds blow with considerable force from December to March and April. These winds, for example, are responsible for carrying moisture in the form of mists to the slopes of the Tilarán mountain range. These mists are what sustain the magnificent cloud forest ecosystem.

Rainfall patterns, although seasonal, vary greatly in intensity across geographical areas. The topography of the country also has a great influence on the weather patterns of a given locality. As a result the timing of the dry and rainy seasons varies a bit on each slope of the mountain ranges that run from the north-west to the south-east and divide the nation into a Caribbean slope and a Pacific slope.

On the Caribbean slope the rainy season begins from mid to late April and continues through December and sometimes January. The wettest months are July and November, with a dry spell that occurs around August or September. Major storms, called "temporales del Atlantico" occasionally buffet this slope between September and February, when it will rain continuously for several days; but an average rainy season day will begin clear with a few hours of sunshine that will give way to clouds and rain by the afternoon. In contrast, the driest months of February and March, might be almost entirely without rainfall.

On the Pacific slope the rainy season begins in May and runs its course until November. Here again, days often begin sunny and pleasant, with rains coming later in the day. This is a period in which the trade winds coming from the north-east are much reduced in intensity, and as a result storms often come in from the Pacific Ocean in September and October. In the northern half of the country the Pacific slope experiences an intense dry season, in which no rain may fall for several months. The forests of the North-West are to a large extent deciduous, letting their leaves fall in order to conserve water. The whole Central Valley, in which the capital is situated, experiences a mild, pleasant dry season that is matched by moderate temperatures for most of the year, and a lower than average amount of rainfall.