Map from Google - Cameroon

In Cameroon, the climate is tropical, semi-arid in the north, and humid and rainy in the rest of the country. There is almost everywhere a dry season in winter and a rainy season in summer; the latter is shorter in the north, while along the coast even in winter there can be some showers. The northernmost part of the country, on the shores of Lake Chad, is the driest area, where less than 600 millimetres of rain per year fall, while the wettest is the coast, where the rainfall exceeds 3,000 mm per year (see the map above).
In the inland southern region lies a plateau, located between 500 and 1,200 metres above sea level, where the altitude mitigates the climate, making the heat more bearable.

In the north the climate is hot and semi-arid, with a rainy season from mid-May to September, while the rest of the year is characterized by hot, often torrid weather, and dryness. In the winter months, a hot wind from the desert blows, the Harmattan, which raises the dust, creating a typical mist; sometimes the wind can reach even the centre and the south. Nights can be cool, sometimes even cold: lows can drop to around freezing in the far north. The northernmost area belongs to the climate zone called Sahel, while south of it lies the zone of the savannah.
North Cameroon
In Maroua, 760 millimetres of rain per year fall, most of which falls from May to September, with a maximum of 235 mm in August. Here is the average precipitation.
Average precipitation - Maroua
Prec. (mm)00120601001952351302500760

August is also the coolest month, the only one in which the average maximum temperatures touches 30 °C, but at this time the air is moist. In the winter months, it gets hot during the day, while nights can be cool; the hottest period runs from March to May, when the temperature often reaches 40 °C, while the highest recorded temperatures in this region are about 47/48 °C. After the rains, in October and November, the torrid heat comes back, albeit less intense, around 35 °C, with peaks of 40 °C. Here are the average temperatures.
Average temperatures - Maroua
Min (°C)171923252523222121211917
Max (°C)323538403835323032353533

Proceeding south, in the south-central plateau, the climate becomes different: the rainy season it's longer, so much so that in Ngaoundéré it runs from April to October, and precipitation amounts to 1,500 mm per year, of which 285 mm in August. Here is the average precipitation.
Average precipitation - Ngaoundéré
Prec. (mm)35451351852252702852501301001540

Compared with the north, in this area the heat is less intense, both because the cloud cover and the early rains which prevent the warming in spring, and because of the altitude: Ngaoundéré is located at 1,100 metres above sea level, and its climate is hot during the day but generally without excesses, around 30 °C in winter and 25 °C in summer, and with cool nights.
Average temperatures - Ngaoundéré
Min (°C)121416171717171716161312
Max (°C)303131302827272526272930

Going further south, in the rest of the plateau of central and southern Cameroon, the total annual rainfall is still around 1,500 mm, but the rainy season is even longer, so as to make possible the growth of the forest. In the capital, Yaoundé, at 750 metres above sea level, 1,500 mm of rain per year fall, but the rains occur from March to November, with two peaks in May and October, interrupted by a relative pause in July and August, when they do not exceed 120 mm per month: in these months, however, the sky is still often cloudy. Here is the average precipitation.
Average precipitation - Yaoundé
Prec. (mm)20451251702001557511523029595201540

Even in the driest period, from December to February, the suns doesn't shine so frequently, not going above 5/6 hours a day. On the positive side: the altitude makes the heat bearable even here, given that the maximum goes from 30 °C in the dry months to 27/28 °C in the rainy months. Here are the average temperatures.
Average temperatures - Yaoundé
Min (°C)202020202020201919192020
Max (°C)303130302928272728282829

Along the coast, a hot and humid weather reigns throughout the year: this is actually the wettest area of Cameroon, where rainfall exceeds 2,000 mm per year: the proximity of the equator, which implies a strong solar radiation, adds to the moisture from the sea, from which heavy rains come in the period May-October, albeit with a decrease in rainfall from mid-June to mid-August in the southern part: in Kribi 2,800 mm of rain per year fall, of which 320 mm in May and even 500 mm in September and October, but "only" 125 mm in July, however, even in December and January, the driest months, about 80 mm of rain fall. So here the climate is almost equatorial, without a real dry season.
The rainiest city of Cameroon is Douala, north of Kribi but still on the coast, where 3,600 mm of rain per year fall, of which more than 500 mm per month from July to September: so here there is no decrease in the mid-summer months; even here in winter there may be some showers, but a little less frequently than in Kribi. Here is the average precipitation.
Average precipitation - Douala
Prec. (mm)3555155240275355680690560405125303605

On the coast, the temperatures are high throughout the year, however, they decrease during the rainy season, when the air is humid, but there are also lively breezes blowing from the sea. Here are the average temperatures.
Average temperatures - Douala
Min (°C)232424242323232323232323
Max (°C)323333323130282829303132

The sea temperature in Cameroon is high throughout the year, as can be seen from the following table.
Sea temperature - Douala
Sea (°C)282929292928272627272828

The coast, as well as the south-central plateau, is never very sunny, even in the driest months.
At a short distance from the coast, we find the highest peak of the country: Mount Cameroon, a volcano which rises up to 4,040 metres; the slopes up to about 2,500 metres are very rainy and are covered by forest, while above 3,000 metres, it's cold all year round.

When to go

The best time to visit Cameroon is winter, from December to February, being the driest and sunniest. In the centre-south it's hot during the day, although it may be a bit cool at night on the plateau. In the arid north it's very hot during the day, while at night it can get a little cold; as early as February the heat can be scorching, so you may prefer to visit the north in December and January.
On the coast, in winter the weather is hot and muggy, with minimum temperatures usually above 20 °C, and highs around 30 °C, but with the breath of the breeze; the sky is often overcast especially in the morning, and there can be some showers, especially in the southern part (see Kribi).

What to pack

In winter: for the north, but also Yaoundé and the south-central plateau, light clothes for the day, loose fitting and of natural fabric (cotton or linen), desert turban for the wind-borne sand and dust, a jacket and a sweater for the evening, comfortable boots or sandals. For the coast, light clothing, a light sweatshirt for the evening. For the top of Mount Cameroon: down jacket, gloves, scarf, hat, raincoat.

In summer: for the north, light clothes, light raincoat or umbrella for the rain showers, light sweatshirt. For Yaoundé and the south-central plateau, light clothes, but also a sweater, and a raincoat. For the coast, light clothing, light sweatshirt, light raincoat or umbrella. For the top of Mount Cameroon: warm jacket, gloves, scarf, hat, raincoat.