The climate of São Tomé and Príncipe is tropical with one long rainy season lasting nine months, from September through May, and producing an annual average rainfall of 900 mm. The rainy season is interrupted by a short period termed, “Gravanito”, and occurs during the months of December and January. During this short period, a decrease in rainfall and average air temperature is observed. The occurrence of this intermediate season is dependent on the Inter-Tropical Convergence Zone. A three-month dry season termed “Gravana” lasts from June through August coinciding with the lowest average temperatures of the year. Mean temperatures vary a few degrees throughout the year, ranging between 22°C and 26°C. The lowest average temperatures occur during the dry season months of June through August, while the rainy season experiences higher temperatures.
- São Tomé and Príncipe has experienced an increase of 1.5°C in average annual temperature between 1951-2010.
- The island of Príncipe has experienced a faster rate of increase in annual average temperature than the southern island of São Tomé.
- Mean annual precipitation decreased between 1951 and 2010 at a rate of 1.7 mm per year.
- March through May seasonal rainfall has significantly decreased since 1960 at an average rate of 10.5 mm per month per decade.
- October through December seasonal precipitation has also decreased, but is not statistically significant. The extent of the dry season, ‘Gravana’ has lengthened in recent decades.