Map from Google - Nigeria

In Nigeria the climate is tropical, semi-arid in the far north, and progressively rainier as you move southward.
In fact, there is a rainy season, due to the African monsoon, which is progressively longer and more intense proceeding from north to south: in the north (see Sokoto, Kano, Maiduguri) it lasts only four months, from June to September, in the centre (see Abuja) it goes from April to October, while in the south (see Lagos, Benin City, Port Harcourt) it goes from March to October, and finally in the south-east, which is the wettest area (see Calabar), it goes from March to November.
Annual precipitation remains below 500 millimetres (20 inches) in the extreme north-east, on the shores of Lake Chad, it ranges from 1,000 to 1,500 mm (40 to 60 in) in the central region, it exceeds 2,000 mm (80 in) in the south, and even 3,000 mm (120 in) in the far south-east (the map above shows the average rainfall, in millimetres on the left and in inches on the right). It follows that the vegetation varies considerably, since we find the semi-arid Sahelian landscape in the north, the savannah in the centre, the forests in the south and the mangroves in the Niger Delta and the coastal areas.

Rainfall in Nigeria

Even the temperatures vary considerably depending on the climatic zones. In the north, winter is warm and dry; it can get uncomfortably hot during the day, even 40 °C (104 °F), but at night it's usually cool, and it can even get cold in the northern hilly areas, where cold records are around freezing (0 °C or 32 °F).
Here are the average temperatures of Kano, located at 500 metres (1,600 feet) above sea level, in the north.
Average temperatures - Kano
KanoJanFebMarAprMayJunJulAugSepOctNovDec
KanoJFMAMJJASOND
Min (°C)121418222322212021191512
Max (°C)303337393834312931343431
Min (°F)545764727372706870665954
Max (°F)86919910210093888488939388

In Kano, precipitation doesn't reach 900 mm (35.5 in) per year. Here is the average precipitation.
Average precipitation - Kano
KanoJanFebMarAprMayJunJulAugSepOctNovDecYear
KanoJFMAMJJASONDY
Prec. (mm)00210701152053101401300865
Prec.(in)000.10.42.84.58.112.25.50.50034.1
Days00016712171010054

In the centre, winter is hot, with highs around 33/35 °C (91/95 °F) in December and January. In this period the Harmattan often blows, a wind able to bring dust from the desert, and to reduce visibility. Here are the average temperatures in Abuja, the capital, located in the centre of the country, at 500 metres (1,600 feet) above sea level.
Average temperatures - Abuja
AbujaJanFebMarAprMayJunJulAugSepOctNovDec
AbujaJFMAMJJASOND
Min (°C)202624252018221818211616
Max (°C)353737363331292930323435
Min (°F)687975776864726464706161
Max (°F)959999979188848486909395

In Abuja, rainfall amounts to 1,200 mm (47 in) per year, including more than 100 mm (4 in) per month from May to October. Here is the average rainfall.
Average precipitation - Abuja
AbujaJanFebMarAprMayJunJulAugSepOctNovDecYear
AbujaJFMAMJJASONDY
Prec. (mm)251065135165220265255105511220
Prec.(in)0.10.20.42.65.36.58.710.4104.10.2048
Days001491214161680082

Even on the coast winter is hot, with highs around 30/32 °C (86/90 °F), and it's also more humid, though tempered by the breezes in the afternoon; here cloudy skies are frequent, at least in the morning, while in the north and centre the winter is sunny. Sometimes, however, the Harmattan can arrive also on the coast.

Harmattan in Lagos

Starting from February, the heat increases in all inland areas, and it becomes torrid in the centre-north from March to May, when temperatures can easily reach 40 °C (104 °F). On the contrary, in the south the increase in temperature is limited, both for the proximity to the ocean, and because the rain showers begin earlier. The rains increase in intensity and frequency and gradually move to the north, until they affect all the country in June. In the area where the wet front advances, clashing with the existing hot and dry air mass, small tornados may form. From June to September, the air is humid and the sky is cloudy throughout the country; temperatures are uniform, and are everywhere around 28/30 °C (82/86 °F): the daytime temperatures are lower than in winter, but relative humidity is higher. In Lagos, a large metropolis, in which the urban heat island effect is noticeable, in addition to moisture coming from the sea and the lagoon, the heat is particularly unpleasant, but at least on the coast the breeze blows. Here are the average temperatures of Lagos.
Average temperatures - Lagos
LagosJanFebMarAprMayJunJulAugSepOctNovDec
LagosJFMAMJJASOND
Min (°C)222424242323222222222322
Max (°C)323333323129282829303132
Min (°F)727575757373727272727372
Max (°F)909191908884828284868890

Only in the far north-east, in the area of Lake Chad, where the rains are not very heavy even in summer, sunny periods, during which the heat returns to be intense, occur quite often.
In the western part of the coast, between mid-July and late August, the rains diminish a bit: in Lagos, June is the wettest month with 310 mm (12 in), then rainfall decreases to 255 mm (10 in) in July and 110 mm (4.3 in) in August, but the sky remains cloudy and moisture high. Here is the average precipitation in Lagos.
Average precipitation - Lagos
LagosJanFebMarAprMayJunJulAugSepOctNovDecYear
LagosJFMAMJJASONDY
Prec. (mm)15407514020531025511016513555201540
Prec.(in)0.61.635.58.112.2104.36.55.32.20.860.6
Days236912161312131151103

In the southeast, the rainiest region, this decrease doesn't occur: in Calabar, where 3,000 mm (120 in) of rain per year fall, rainfall remains around 400 mm (16 in) per month from June to September.
The rains cease on mid-September in the far north and during the month of October in the centre, while they linger on until November in the far south. Along with sunny weather, hot and dry weather conditions return in the centre-north, while in the south the humidity remains higher because of the influence of the sea.
In the mountainous area, on the border with Cameroon, where the Gashaka Gumti National Park is located, and the highest peak is Chappal Waddi, 2,419 metres (7,936 feet) high, it's cool above 1,500 metres (5,000 feet). Another area with some elevations is the Jos Plateau, in the centre of the country, where the temperature is fairly mild: in Jos, the capital of the Plateau State, located at 1,200 metres (3,900 feet), during the day the temperature goes from 28 °C (82 °F) in January, to 31/32 °C (88/90 °F) in March and April, to 24/25 °C (75/77 °F) in July and August.
The altitude enhances atmospheric instability in the rainy months, so the rains are more abundant than in surrounding areas, and they go from 1,300 mm (51 in) per year in the state capital, to more than 1,500 mm (60 in) in the rest of the plateau.
Average temperatures - Jos
JosJanFebMarAprMayJunJulAugSepOctNovDec
JosJFMAMJJASOND
Min (°C)141618191818171717171614
Max (°C)283032312927252427292928
Min (°F)576164666464636363636157
Max (°F)828690888481777581848482

In the far south-east of Nigeria, the Cross River National Park is another hilly area, even wetter and covered by forests.

Zuma Rock

For swimming, the sea is warm all year round, although care must be taken with regard to the currents. The amount of sunshine, however, is never great on the coast, even in the dry months.
Sea temperature - Lagos
LagosJanFebMarAprMayJunJulAugSepOctNovDec
LagosJFMAMJJASOND
Sea (°C)282829292928262526272928
Sea (°F)828284848482797779818482

When to go

The best time to visit Nigeria is winter, from November to February, and in particular the months of January and February, which are the driest and sunniest. It's true that it can get hot during the day almost everywhere, and especially in the centre and north, while at night it can even get cold in the far north, and sometimes the Harmattan can bring sand and dust storms, but at least you avoid the scorching heat of the period from February to May, as well as the summer rains. Even in November you can find this kind of hot and dry weather, except in the south-east (see Calabar), where it still rains. Even February is similar, but torrid days start to be more frequent in inland areas, at some distance from the sea.

What to pack

In winter: in the north, Jos and Abuja, light clothes for the day, long and made of natural fibres (cotton or linen), desert turban for the wind-borne sand and dust, a jacket and a sweater for the evening, comfortable and breathable shoes. In the south and Lagos, light clothing, a light sweater for the evening and air conditioned places, light scarf for the dusty wind.
In summer: lightweight clothing of natural fabrics (cotton or linen), light raincoat or umbrella for the rain. In Jos, you can add a sweatshirt and a light jacket. For women, it is best to avoid shorts and miniskirts, especially in the north.