Map from Google - Congo

In the Republic of Congo, the climate is equatorial (hot and humid with no dry season) in the north, and tropical (hot and humid with a dry and a rainy season) in the center and south.
In the north (see Ouésso, Impfondo), the temperatures are stable during the year, however, they decrease slightly in July and August. Here are the average temperatures of Ouésso, located in the north, at 1,120 feet of altitude.
Average temperatures - Ouesso
OuessoJanFebMarAprMayJunJulAugSepOctNovDec
OuessoJFMAMJJASOND
Min (°F)686870707070707070707068
Max (°F)889090908886848486868686

In the north there are two seasons which are only relatively dry, December to February and June to August; in addition, the more we go to the north, the more evident is the first (the one that goes from December to February), because we are moving away from the equator and entering the area where the rainfall regime is typical of the Northern Hemisphere (ie with a dry winter). The result is that the north has a better distributed rainfall, so it's covered by the forest, while the central and southern areas are covered by savanna.
Here is the average precipitation in Ouesso.
Average precipitation - Ouesso
OuessoJanFebMarAprMayJunJulAugSepOctNovDecYear
OuessoJFMAMJJASONDY
Prec. (in)2.23.15.95.15.94.53.16.18.78.75.93.362.6
Days45981196101314116106

In the center-south, in the tropical region, the dry season runs from June to August in the center, from June to September in the south, and from mid-May to mid-October on the coast. The rainfall amount ranges from 60 to 80 inches per year in the north, while in the south it decreases near the coast (see Pointe Noire), where it descends to 47 inches. In fact, in the winter months the sea is cooled by the cold Benguela Current, which inhibits almost completely the rains for a longer period than elsewhere in the country; on the other hand, this current generates fogs and low clouds that reduce insolation.
Here is the average precipitation in Pointe Noire.
Average precipitation - Pointe Noire
Pointe NoireJanFebMarAprMayJunJulAugSepOctNovDecYear
Pointe NoireJFMAMJJASONDY
Prec. (in)5.98.38.97.13.5000.10.52.86.55.749.3
Days10101211500141012984

In inland areas, the temperatures are high throughout the year, and the air is hot and muggy: the daily average ranges from 77 °F in January, to 79 °F in February and March, to 75 °F in July and August, while there may be a little cool at night from June to August in the hills of the Plateaux Department, around 61/623 °F.
On the coast, the temperature becomes a little cool during the day from June to September, when the maximum ranges between 77 °F, while at night they remain quite high, around 64/68 °F, due to the mitigating effect of the sea, as we can see from the averages of Pointe Noire.
Average temperatures - Pointe Noire
Pointe NoireJanFebMarAprMayJunJulAugSepOctNovDec
Pointe NoireJFMAMJJASOND
Min (°F)757575757368646670737375
Max (°F)868688868481777779828484

In the capital, Brazzaville, which is situated in the south-east along the Congo, the river that gives its name to the country, the daytime temperatures are about 86 °F from September to January, they go up to 88/90 °F from February to April, and they drop around 81/82 °F from June to August. Here are the average temperatures.
Average temperatures - Brazzaville
BrazzavilleJanFebMarAprMayJunJulAugSepOctNovDec
BrazzavilleJFMAMJJASOND
Min (°F)707072727064636468707070
Max (°F)868888908682818286868684

Throughout the year, 53 inches of rain fall in Brazzaville, with two maxima in Aprile and November, while it hardly ever rains from June to September. Here is the average precipitation.
Average precipitation - Brazzaville
BrazzavilleJanFebMarAprMayJunJulAugSepOctNovDecYear
BrazzavilleJFMAMJJASONDY
Prec. (in)4.74.97.38.35.30.100.10.15.58.97.752.9
Days981013800039131285

North of Brazzaville, on the Koukouya Plateau, it rains more than in the capital, around 70/80 inches per year, with two maxima in March-April and October-November.
The amount of sunshine is never good throughout Congo, and it does not get better in the dry season, on the contrary it slightly worsens if possible, when even in the interior the sky is often cloudy, while in the period from October to April, between the rains the sun comes out.

Congo, elephants

For swimming, the sea is warm from October to May, while it gets a little cool, but still acceptable, in July and August, when it drops to 72 °F.
Sea temperature - Pointe Noire
Pointe NoireJanFebMarAprMayJunJulAugSepOctNovDec
Pointe NoireJFMAMJJASOND
Sea (°F)818284827975727275798181

When to go

The best time to visit the Republic of Congo runs from June to August, being the coolest and the driest, except in the north where it rains even during this period; however, even though it's not too hot, moisture is high and the sky is cloudy even in the center and south where it's the dry season; remember that nights can be cool on the plateau, and even on the coast it can get a little cool, at least for swimming, also considering the lack of sunshine.
In August, at the end of the dry season, the animals of the savanna gather in the few remaining waterholes or along rivers, and therefore it's more easy to watch them.

What to pack

All year round, light clothes, of natural fibers, a light sweatshirt for the evening, umbrella or light raincoat for showers during the rainy season. From June to September, you can add a sweatshirt or sweater and a light jacket, especially in the south and along the coast.