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 inland areas, with a short dry season from June to August, while in the central and southern part of the 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 temperature varies 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 (86 °F), while from June to September it drops a bit, hovering around 24/25 °C (75/77 °F) in the central and southern part of the coast, and to 27/28 °C (81/82 °F) in inland areas.
The annual precipitation in most of the country ranges from 1,500 to 2,000 millimetres (60 to 79 inches), while it's close to 3,000 mm (118 in) in the northern part of the coast, where the capital Libreville 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.
Much 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 not 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 (2,000 feet) above sea level.
Average temperatures - Bitam
Min (°C)202020202020191920202020
Max (°C)293030303028272728292929
Min (°F)686868686868666668686868
Max (°F)848686868682818182848484

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
Prec. (mm)50701801902301002540275300210751745

In Bitam the sun does not shine very often, and indeed the least sunny months, when the sky is normally cloudy, are the driest, July and August. Things are a bit better in the other dry season, from December to February.
Sunshine - Bitam
Sun (hours)455554323444

The capital Libreville is located on the northern part of the coast. Here are the average temperatures.
Average temperatures - Libreville
Min (°C)242323232423222223232324
Max (°C)303131313029282829293030
Min (°F)757373737573727273737375
Max (°F)868888888684828284848686

In Libreville, 2,600 mm (102 in) 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 - Libreville
Prec. (mm)205290265395245401111053604152602592

In Libreville, the amount of sunshine is all in all decent from December to May, even though it is a rainy season, while in the dry period, especially in July and August, the sky is often cloudy.
Sunshine - Libreville
Sun (hours)666654433445

In the south, the winter cooling is more noticeable, so much so that maximum temperatures drop to 26/27 °C (79/81 °F) in July and August. Here are the average temperatures of Tchibanga.
Average temperatures - Tchibanga
Min (°C)222222222220181921222222
Max (°C)303131323028272628293029
Min (°F)727272727268646670727272
Max (°F)868888908682817982848684

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
Prec. (mm)195190220175100614301753001801585

In the south the sun is even rarer, and here too the least sunny season is the dry one, when the sky is normally cloudy.
Sunshine - Tchibanga
Sun (hours)555543322344

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 (77 °F) at Libreville, and around 23 °C (73 °F) in the central and southern part of the coast.
Sea temperature - Libreville
Sea (°C)282929292826252626272728
Sea (°F)828484848279777979818182

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 have 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, along the coast the breeze can cause a cool feeling, especially in the southern part.

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 central and southern part of the coast.