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Of the weather parameters, rainfall is the most variable. Islandwide, during the period 1951 to 1980, annual rainfall ranged from a maximum of 2593 millimetres (102.09 inches) in 1963 to a minimum of 1324 millimetres (52.13 inches) in 1976 with an average of 1940 millimetres (76.38 inches) annually. The hundred-year (1881-1990) mean annual rainfall is 1895 millimetres (74.61 inches). Historically, the wettest year on record was 1933 with an annual rainfall of 2690 millimetres (116.54 inches) whilst the driest year was 1920 with an annual rainfall of 1299 millimetres (51.14 inches).

Some mountainous areas in the island's northeast receive more than 5080 millimetres (200 inches) annually, whilst coastal areas to the southeast and south-centre receive less than 889 millimetres (35 inches) annually.

Yearly, most areas of the island have two distinct wet seasons, May to June and September to November; these wet seasons occur as regularly yearly cycles.

Most of the rainfall during the May to June period is as a result of the periodic march of solar radiation intensity, which peaks at that time. During the period September to November the rainfall is more directly the result of the lifting and movement of the sub-tropical high-pressure cell in the Atlantic Ocean. Such behaviour deepens the easterly Trades, allowing instability zones to develop; as such, a significant portion of the rainfall during this period is produced by upper- and low-level troughs, tropical waves, tropical depressions, tropical storms and hurricanes.

The driest period is usually December to March. Most of the rainfall during this period is associated with cold fronts migrating from North America.

Whether during the dry or rainy season, however, other rain-producing systems are influenced by the sea breeze and orographic effects which tend to produce short-duration showers, mainly during mid-afternoon.