Map from Google - Gabon

In Gabon, an African country crossed by the equator, the climate is hot and humid all year round in the north and in the inland areas, with a short dry season from June to August, while in the south-central coast, the temperatures become cooler during the dry season. In the far south the dry season is even longer, going from mid-May to mid-September.
The temperatures vary little over the course of the year in the north and centre of the country: the warmest period is from January to May, when highs are around 30 °C, while from June to September it drops a bit, hovering around 24/25 °C on the south-central part of the coast, and to 27/28 °C in the inland areas.
The annual precipitation in most of the country ranges from 1,500 to 2,000 millimetres, while it's close to 3,000 mm in the northern part of the coast where the capital Libréville is located. In the northern inland areas, near the equator (see Bitam, Oyem, Makakou), there are two rainy seasons, from March to May and from September to November, related to the two passages of the sun at its zenith.
The majority of Gabon is occupied by forests and mangroves, except the extreme east, which is occupied by the savannah. There are several national parks, designed to protect the biodiversity of the region.
The amount of sunshine is never great, and it's even worse in the dry season, from June to August, when the sky is often cloudy. The relatively sunnier period runs from December to February.
In the interior there are hilly areas, where the heat is tempered by altitude. Here are the average temperatures of Bitam, situated in the far north, at 600 metres above sea level.
Bitam average temperatures
Bitam J F M A M J J A S O N D
Min (°C) 20 20 20 20 20 20 19 19 20 20 20 20
Max (°C) 29 30 30 30 30 28 27 27 28 29 29 29

As mentioned, in the north there are two rainy seasons, and as a result there will be two relatively dry periods, from December to February and from June to August. Here is the average rainfall in Bitam.
Average precipitation Bitam
Bitam J F M A M J J A S O N D Year
Prec. (mm) 50 70 180 190 230 100 25 40 275 300 210 75 1745
Days 4 5 11 14 14 8 4 4 15 20 14 7 120

The capital Libréville is located on the northern part of the coast. Here are the average temperatures.
Libréville average temperatures
Libréville J F M A M J J A S O N D
Min (°C) 24 23 23 23 24 23 22 22 23 23 23 24
Max (°C) 30 31 31 31 30 29 28 28 29 29 30 30

In Libréville, 2,600 mm of rain per year fall, with a single dry season from June to August, when it almost never rains. Here is the average rainfall.
Average precipitation Libréville
Libréville J F M A M J J A S O N D Year
Prec. (mm) 205 290 265 395 245 40 1 11 105 360 415 260 2592
Days 14 15 17 19 15 3 0 5 13 22 21 16 160

In the south, the winter cooling is more noticeable, so much so that the maximum temperatures drop to 26/27 °C in July and August. Here are the average temperatures of Tchibanga.
Tchibanga average temperatures
Tchibanga J F M A M J J A S O N D
Min (°C) 22 22 22 22 22 20 18 19 21 22 22 22
Max (°C) 30 31 31 32 30 28 27 26 28 29 30 29

In the south the dry season is longer, so that it begins in mid-May and lasts until September. Here is the rainfall in Tchibanga.
Average precipitation Tchibanga
Tchibanga J F M A M J J A S O N D Year
Prec. (mm) 195 190 220 175 100 6 1 4 30 175 300 180 1585
Days 14 14 16 15 9 1 1 1 4 16 21 15 127

White Canyon, Gabon

For swimming, the sea is warm all year round, even though the water temperature drops a little from July to September, when it goes down to around 25 °C at Libréville, and around 23 °C in the south-central part of the coast.
Sea temperature Libréville
Libréville J F M A M J J A S O N D
Sea (°C) 28 29 29 29 28 26 25 26 26 27 27 28

When to go

The best time to visit Gabon is from June to mid-September, and in particular the months of July and August, because they are the least rainy, and also the ones in which in the past there has been no anomalies in the rains; during this time the heat is not excessive, but the humidity is high and the sky is often cloudy. Nights can be a little cool in inland areas, while during the day it can be a little cool along the coast, when the breeze blows.

What to pack

All year round, light clothes, of natural fibres, a light sweatshirt for the evening, umbrella or light raincoat during the rainy season. From June to September, you can add a sweatshirt and a light jacket, especially on the south-central part of the coast.