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 24 inches of rain per year fall, while the wettest is the coast, where the rainfall exceeds 120 inches per year (see the map above).
In the inland southern region lies a plateau, located between 1,600 and 3,900 feet 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 center 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, 30 inches of rain per year fall, most of which falls from May to September, with a maximum of 9.2 inches in August. Here is the average precipitation.
Average precipitation - Maroua
Prec. (in)0000,82,43,97,79,35,110029,9

August is also the coolest month, the only one in which the average maximum temperatures touches 86 °F, 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 105 °F, while the highest recorded temperatures in this region are about 117/118 °F. After the rains, in October and November, the torrid heat comes back, albeit less intense, around 95 °F, with peaks of 105 °F. Here are the average temperatures.
Average temperatures - Maroua
Min (°F)636673777773727070706663
Max (°F)909510010410095908690959591

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 in a year 60 in of rain fall, of which 11 inches in August. Here is the average precipitation.
Average precipitation - Ngaoundéré
Prec. (in)0,10,21,85,37,38,910,611,29,85,10,4060,6

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 3,600 feet above sea level, and its climate is hot during the day but generally without excesses, around 86 °F in winter and 77 °F in summer, and with cool nights.
Average temperatures - Ngaoundéré
Min (°F)545761636363636361615554
Max (°F)868888868281817779818486

Going further south, in the rest of the plateau of central and southern Cameroon, the total annual rainfall is still around 60 in, but the rainy season is even longer, so as to make possible the growth of the forest. In the capital, Yaoundé, at 2,450 feet above sea level, 60 in 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 4.7 inches per month: in these months, however, the sky is still often cloudy. Here is the average precipitation.
Average precipitation - Yaoundé
Prec. (in)0,81,84,96,77,96,134,59,111,63,70,860,6

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 highs go from 86 °F in the dry months to 81/82 °F in the rainy months. Here are the average temperatures.
Average temperatures - Yaoundé
Min (°F)686868686868686666666868
Max (°F)868886868482818182828284

Along the coast, a hot and humid weather reigns throughout the year: this is actually the wettest area of Cameroon, where rainfall exceeds 78 inches 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 110 in of rain per year fall, of which 12.5 inches in May and even 20 inches in September and October, but "only" 4.9 inches in July, however, even in December and January, the driest months, about 3 in 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 142 in of rain per year fall, of which more than 20 inches 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. (in)1,42,26,19,410,81426,827,22215,94,91,2141,9

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 (°F)737575757373737373737373
Max (°F)909191908886828284868890

The sea temperature in Cameroon is high throughout the year, as can be seen from the following table.
Sea temperature - Douala
Sea (°F)828484848482817981818282

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 13,255 feet; the slopes up to about 8,200 feet are very rainy and are covered by forest, while above 10,000 feet, 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 center-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 lows usually above 70 °F and highs around 85 °F, 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.