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São Tomé month by month — at a glance
Hover each month for the summary. Click to pin it.
01 Twelve hours of daylight, four and a half of sun
Key moments at a glance
No rain at all in the whole month, with 161.2 hours of sun and 27.3 °C.
165 hours of sun, the year's peak, with only 19 mm of rain.
114.7 hours of sun, the year's minimum, with 110 mm over 12 days.
131 mm over 12 days, the rainfall peak, with 30.2 °C highs.
The reason is cloud, and here it is almost permanent. The island is a volcanic cone in the middle of the Gulf of Guinea, and the humid ocean air is forced to rise on meeting the relief, condenses and forms cloud. The result is a sky that rarely opens fully.
The monthly distribution is also very flat: the sunniest month is June, with 165 hours, and the least sunny October, with 114.7. Not even the year's best month reaches 5.5 hours a day. For anyone travelling expecting a bright equatorial destination, it is the first correction worth making.
02 The gravana
In July it does not rain. Zero millimetres in the whole month, on an island that receives 885 a year.
What does organise the year is rain, and it has a local name: the gravana is the dry season, running roughly from June to September. The figures are emphatic: 19 millimetres in June, 0 in July, 1 in August and 17 in September. In July, on average, no water falls in the whole month.
The rest of the year is the opposite. From October to May precipitation runs from 81 to 131 millimetres a month, with between 8 and 12 rain days. The annual total is 885 millimetres over 94 days, and practically all of it falls outside the dry block.
There is a nuance worth grasping: the gravana is dry but not especially sunny. June and July are indeed the two sunniest months — 165 and 161.2 hours — but that is still barely over five hours a day. The dry season does not bring blue skies: it brings an absence of rain.
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03 The humidity that never drops
The third variable is the one least looked at and the one most felt physically. Mean annual relative humidity is 82 %, and the range is extremely narrow: from 77 % in July to 85 % in January, November and December. There is no month in which the air is dry.
Combined with temperature, that humidity explains how it actually feels. The mean high runs from 27.3 degrees in July to 30.2 in March, and the low from 20.4 to 22.6. Those are moderate figures on paper, but with eight parts in ten of the air saturated with vapour, the body reads them considerably higher.
The records confirm how narrow the range is: the absolute maximum in the series is 33.9 degrees and the minimum 13.4, the latter recorded in August and probably at altitude. It is a climate without thermal surprises and without respite from humidity.
04 Which month to pick
The good window is the gravana, and within it July and August are the best months. July gives 0 millimetres of rain, 161.2 hours of sun, 27.3 degrees by day and humidity at its annual minimum of 77 %. August gives 1 millimetre, 148.8 hours of sun, 27.7 degrees and 78 % humidity.
June is the third option and has an argument of its own: it is the sunniest month of the year, at 165 hours, with 19 millimetres of rain over 3 days. If light matters to you more than absolute dryness, it is the one to mark.
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The months to avoid are March, April, October and November: 131, 122, 110 and 99 millimetres over 11 or 12 rain days, and with sunshine in the year's lowest stretch — 126 hours in April, 114.7 in October. December, January and February sit in between: it rains, but over fewer days and with more light.
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