Map from Google - Democratic_Republic_Congo

In the Democratic Republic of the Congo (formerly Zaire), huge central African country, the climate is equatorial, that is, hot and humid all year round with no dry season, in the central area crossed by the equator where there is a vast rain forest (the second in the world after the Amazon), or tropical, that is hot all year round but with a dry season, usually of short duration, in the areas north and south of the equator. The dry season occurs in opposites periods, from December to February (boreal winter) north of the equator, and from June to September (austral winter) in the southern part.
The rains occur in the form of downpour or thunderstorm (the Congo is the country that receives the greatest number of lightning strikes in the world), usually in the afternoon, all year round in the equatorial zone, and in the long rainy season in both the tropical areas.
The country is largely occupied by the huge basin of the Congo River and its tributaries.
Most of the country is located at an altitude of a few hundred metres (between 300 to 800 metres, but in some areas even from 800 to 1,300 metres, as in the Southeast), which are often sufficient to mitigate the climate, while in the easternmost part, near the Rift Valley lakes, there are also high mountain ranges.

Equatorial climate

Congo, Equatorial climate zone

In the equatorial zone, temperatures are stable throughout the year.
Here are the average temperatures of Kisangani, located near the equator, at 400 metres above sea level.
Average temperatures - Kisangani
KisanganiJanFebMarAprMayJunJulAugSepOctNovDec
KisanganiJFMAMJJASOND
Min (°C)201920202019191919202020
Max (°C)313131313130292930303030

The rains in the equatorial zone are abundant, and hover around 1,700/2,000 millimetres per year. Rainfall experiences two maxima, at the two passages of the sun at its zenith (late March and late September), and given the inertia of the air masses, the rainiest months are usually April-May and October-November. The least rainy periods are usually January-February and June-July, in which, however, usually more than 100 mm of rain fall per month.
This is the average rainfall in Kisangani.
Average precipitation - Kisangani
KisanganiJanFebMarAprMayJunJulAugSepOctNovDecYear
KisanganiJFMAMJJASONDY
Prec. (mm)901151651801601151151551852302151651890
Days9913151387914171512141

Tropical climate

Congo, areas with tropical climate

To the north and south of the equatorial zone (respectively, zone 1 and 2 in the map), the climate becomes tropical, because there is a clear dry season. The rainfall in the tropical climate zones averages normally between 1,000 and 1,700 mm per year. Even the temperatures vary more, and are lower in the winter dry season, and higher in the summer rainy season, but especially during spring, at the end of the dry season.
Here are the average temperatures of Gemena, in the north-west (Zone 1), at 400 metres above sea level.
Average temperatures - Gemena
GemenaJanFebMarAprMayJunJulAugSepOctNovDec
GemenaJFMAMJJASOND
Min (°C)192020212120202020201919
Max (°C)313332313130292930303031

Here, 1,650 mm of rain fall annually, with a maximum in the period from August to October. The dry season is short and lasts from December to February, a time when some showers are still possible.
Here is the average precipitation.
Average precipitation - Gemena
GemenaJanFebMarAprMayJunJulAugSepOctNovDecYear
GemenaJFMAMJJASONDY
Prec. (mm)3555125145165155180215195215120451650
Days461112141212151617116136

In the tropical zone of the southern hemisphere (zone 2), both the temperatures and the rains have a reversed trend.
The capital Kinshasa is located in the west, at about 300 metres above sea level, and 4 degrees south latitude. Temperatures are high and fairly stable throughout the year, but in the austral winter (June to August) they become more pleasant.
Here are the average temperatures.
Average temperatures - Kinshasa
KinshasaJanFebMarAprMayJunJulAugSepOctNovDec
KinshasaJFMAMJJASOND
Min (°C)222222222219181921222222
Max (°C)303131313028272830303030

Rainfall in Kinshasa amounts to 1,400 millimetres per year. Within the rainy season from October to May, you can see the two peaks corresponding with the passages of the sun at its zenith (April and November).
Here is the average precipitation.
Average precipitation - Kinshasa
KinshasaJanFebMarAprMayJunJulAugSepOctNovDecYear
KinshasaJFMAMJJASONDY
Prec. (mm)155135170195125714351352551701387
Days10101315111014111614106

Tropical climate with long dry season

Congo, area having a tropical climate with a long dry season

In the short coastline overlooking the Atlantic Ocean, rainfall drops below 1,000 mm per year, because in the winter months the cold Benguela current, which inhibits the formation of rainy clouds, arrives here, but on the other hand it is able to bring some cool air, and to create fog and low clouds.
Here are the average temperatures of Banana, on the coast.
Average temperatures - Banana
BananaJanFebMarAprMayJunJulAugSepOctNovDec
BananaJFMAMJJASOND
Min (°C)222222232220181921232323
Max (°C)303030302825252526272828

In this area, rainfall decreases even to about 800 millimetres per year; the rains are not particularly abundant, except perhaps in April.
Here is the average precipitation.
Average precipitation - Banana
BananaJanFebMarAprMayJunJulAugSepOctNovDecYear
BananaJFMAMJJASONDY
Prec. (mm)701251301706000052012080780
Days589125000157658

The sea temperature is high in the summer months, while it becomes a bit cool in the winter months, down to 22 °C in July and August. However, this part of the sea, apart from the socio-political conditions of the country, does not lend itself to beach tourism, because of frequent cloud cover.
Here are the average sea temperatures.
Sea temperature - Banana
BananaJanFebMarAprMayJunJulAugSepOctNovDec
BananaJFMAMJJASOND
Sea (°C)272829282624222223252727

The country is more extended to the south than to the north (it stretches front the 13th parallel south to the 5th parallel north), and in the south there are often places located at higher elevations. The result is that in some areas the austral winter is more pronounced, dry and sunny, but very cool or even cold at night.
Here are the average temperatures of Lubumbashi, located in the far south, at 1,300 metres of altitude. Here, in winter (June to mid-August) at night the temperature can reach even zero degrees. Additionally, you notice the temperature increase in the second half of the dry season, between mid-August and October.
Here are the average temperatures.
Average temperatures - Lubumbashi
LubumbashiJanFebMarAprMayJunJulAugSepOctNovDec
LubumbashiJFMAMJJASOND
Min (°C)161616141088913151616
Max (°C)262626272625252730312826

In Lubumbashi, the winter dry season is very evident (it practically never rains from May to September) and long (lasting more than six months). Precipitation amounts to 1,200 mm per year, with a maximum in summer (December to February) when the rains are abundant (they exceed 250 mm per month).
Here is the average precipitation.
Average precipitation - Lubumbashi
LubumbashiJanFebMarAprMayJunJulAugSepOctNovDecYear
LubumbashiJFMAMJJASONDY
Prec. (mm)2552552606050005351652551240
Days24222092000151724124

Mountains

Congo, area with a mountain climate

In the eastern part of the Democratic Republic of the Congo, there are the mountain ranges, related to the East African Rift. The Rift has also produced a number of lakes, such as Lake Albert (located at 615 metres above sea level), Lake Edward (915 m), Lake Kivu (1,465 m), Lake Tanganyika (770 m), and Lake Mweru (920 m).
The mountains have an influence on precipitation, making rainier the western side, and isolating the eastern part from the humid currents, so that in the central and southern shores of Lake Tanganyika the rainfall drops below 1,000 mm per year.
Here are the average temperatures of Goma, located at 1,500 metres above sea level, on the shores of Lake Kivu. Although we are at the equator, at this altitude the temperatures are pleasant all year round, and cool at night.
Average temperatures - Goma
GomaJanFebMarAprMayJunJulAugSepOctNovDec
GomaJFMAMJJASOND
Min (°C)141414141413131313131313
Max (°C)252525252525252525252524

The rainfall amounts to 1,250 millimetres per year; from June to August the rains are fairly rare and not abundant.
Here is the average precipitation.
Average precipitation - Goma
GomaJanFebMarAprMayJunJulAugSepOctNovDecYear
GomaJFMAMJJASONDY
Prec. (mm)115951101401354530651151501451201265
Days171720221775814202320190

The most imposing mountain range of the country are the Ruwenzori mountains, which are located in the far east, on the border with Uganda, almost at the equator. The highest mountain is Mount Stanley (in Margherita Peak, 5,109 metres), where there is also a glacier. The mountains are very wet, since they receive constant rain and are often covered with clouds, and are home to a variety of flora and fauna, which change with altitude: between 2,000 and 3,000 metres there is a montane forest; above 3,000 metres there are species that must withstand night frosts as well as the strong solar radiation during the day, and above 4,000 metres a moorland similar to the alpine one. The Ruwenzori mountains in the Congolese side are protected in the Virunga National Park (World Heritage Site), which extends to the south, beyond Lake Edward, to the other imposing mountain range, always on the border with Uganda, exactly the Virunga Mountains.
West of Lake Kivu we find the Mitumba mountains, whose highest peaks are Mount Kahuzi (3,308 metres) and Biega (2,790 metres), protected in the Kahuzi-Biega National Park, UNESCO World Heritage Site, however, declared at risk because of illegal hunting and poaching.
Further south of Virunga there are the Itombwe mountains, which culminate in Mount Mohi, 3,475 metres high.

Giant Lobelia, Ruwenzori mountains

The amount of sunshine in much of the Democratic Republic of Congo is never very good, because the strong radiation often causes daytime cloudiness, most likely starting at noon and in the afternoon. In the early morning instead, mists often occur, due to moisture evaporating from the ground. On the coast instead fog and low clouds often form, due to the cool sea current. The situation is different in the southernmost part (see Lubumbashi), where the dry season is more pronounced, and during the austral winter (May to September) the sky is often clear.
The equatorial sun is still very strong, and requires adequate protection even when the sky is cloudy, especially at high altitudes.

When to go

There is no single best time to visit the Democratic Republic of the Congo, as the dry season comes at different times: in the tropical region north of the equator it runs from December to February; in the tropical part south of the equator (see Kinshasa) it goes from June to August; in the area further south (see Lubumbashi), there is little rain even in May, September and October, although in the latter two months it can get hot during the day, especially below one thousand metres (see for instance the Kundelungu National Park).
In the equatorial zone, where it rains throughout the year, there is usually a drop in rainfall in January-February and June-July.

What to pack

In the equatorial zone, all year round, light clothing, of natural fibres, a light sweatshirt for the evening, umbrella or light raincoat for the rain.
In tropical areas, in the rainy season, umbrella or light raincoat for the rain; in the relatively cool dry season (December to February in the north, June to September in Kinshasa and the south) you can add a sweatshirt and a light jacket, especially in the south and on the coast.
At low mountain elevations, above a thousand metres (see Lake Kivu and Goma, Lubumbashi) you can bring a sweatshirt and a light jacket for the evening, all year round.
In the far south, above a thousand metres (see Lubumbashi), from May to August you can add sweater, warm jacket and hat for nights and early mornings.
For the tops of the highest mountains, like Ruwenzori, all year round, winter clothes, jacket, hat, gloves.