Rainy Season Around the Corner, Weatherman Predicts
The Instituto Meteorológico Nacional (IMN) - the national weatherman - announced yesterday that the country has begun its transition from the "dry" to the "wet" season.

Typically the rainy season does not start until the middle of May, however, if the forecast is precise, Gustavo Murillo, said that the rainy season is upon us evidenced by the cloudy skies and isolated showers over the last couple of days.

The clouds are isolated downpours are expected to continue today and for the balance of the week.

The meteorologist  added that the the country is facing an atmospheric phenomenon  that has brought isolated heavy downpours in places like Alajuela, Heredia, Cartago and San José.

A number of low pressure systems in the Pacific and near the Panama border and south Caribbean.

On Friday, Limón received only 2.8 millimetres (0.11 inches) or rain, while at the Pavas airport in San José, 26.6 millimetres (1.05 inches) were reported.

The passing from dry season to wet season results from the interaction of the vientos alisions - trade winds - with local topography.

When moisture-laden air coming in off the Caribbean Sea encounters the coastline, the difference in surface temperature between the land and the water can often trigger showers.

Moving further inland the air reaches the eastern foothills of the country's cordillera (mountainous backbone).

As the air mass rises to pass over the barrier, it cools, and because cool air can hold less moisture than warm air, it rains, causing the middle elevations of the Caribbean-facing slopes to be the wettest areas in the country.

From mid-November through mid-May (on average) the Central Valley and the northwestern portion of the country are affected by an annual dry season.

The warm moist air driven westward by the trade winds lose its moisture as it crosses the cordilleras and the resulting dry air gusts down the Pacific slopes drying out everything in its path.

With such low moisture content, few clouds form to block the sunshine and the prevailing winds keep Pacific breezes from bringing moisture onshore, thus, further promoting the dryness.

As the trade wind belt moves northward in response to global climatic conditions (principally, the angle of the sun and area of greatest surface heating), Costa Rica enters its rainy season as moist air flows in from both oceans and convection currents cause showers to occur.

Regional weather conditions, such as tropical waves, tropical depressions, and even hurricanes farther north and east in the Caribbean, can greatly affect precipitation levels here.

The first two atmospheric phenomena usually bring increased rainfall to the eastern side of the country when they pass through the western portion of the Caribbean Sea.

Distant hurricanes (fortunately these major storms almost never reach Costa Rica - one hit south of Limón in 1910) can result in what are known here as temporales del Pacífico.

The change is characteristic for the month of April, defined by the isolated downpours.

Murillo added that the weather this month is very unstable and it is difficult to make predictions, saying only that for the coming week the mornings should be sunny with showers or downpours in the afternoon, typical of the weather during the past week.