Map from Google - Nigeria

In Nigeria the climate is tropical, semi-arid in the far north, and progressively rainier as one moves south.
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 four months from June to September, in the center (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.
The annual precipitation remains below 20 inches in the extreme north-east, on the shores of Lake Chad, it ranges from 40 to 60 inches in the central region, it exceeds 80 inches in the south, and even 120 inches in the far south-east (the map above shows the average rainfall, in millimeters on the left and inches on the right). It follows that the vegetation varies considerably, since it goes from the semi-arid Sahelian landscape in the north, the savannah of the center, the forests of the south and the mangroves of 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 105 °F, but at night it's usually cool, and it can get even cold in the northern hilly areas, where the cold records are around freezing.
Here are the average temperatures of Kano, located at 1,600 feet above sea level, in the north.
Average temperatures - Kano
Min (°F)545764727372706870665954
Max (°F)86919910210093888488939388

In Kano, the annual rainfall is below 35 inches per year. Here is the average precipitation.
Average precipitation - Kano
Prec. (in)

In the center, winter is hot, with highs around 91/95 °F in December and January. In this period the prevailing wind, the Harmattan, brings dust from the desert, reducing visibility. Here are the average temperatures in Abuja, the capital, located in the center of the country, at 1,600 feet above sea level.
Average temperatures - Abuja
Min (°F)687975776864726464706161
Max (°F)959999979188848486909395

In Abuja, rainfall amounts to 48 inches per year, including more than 4 inches per month from May to October. Here the average rainfall.
Average precipitation - Abuja
Prec. (in)

On the coast, the winter is still hot, with highs around 86/90 °F, it's more humid, but tempered by the afternoon breezes; here cloudy skies are frequent, at least in the morning, while in the north and center 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 hot in the center-north from March to May, when highs can easily reach 105 °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, small tornados may form. From June to September, the air is humid and the sky is cloudy throughout the country; the temperatures are uniform, so that they are everywhere around 86/90 °F: the daytime temperatures are lower than in winter, but air humidity is higher. In Lagos, a large metropolis affected by the urban heat island effect, in addition to the humidity 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
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 12.3 in of rain, then the rainfall decrease to 10.1 inches in July and 4.4 inches in August, but the sky remains cloudy and moisture high. Here is the average precipitation in Lagos.
Average precipitation - Lagos
Prec. (in)0.61.635.58.112.2104.

In the southeast, the rainiest region, this decrease doesn't occur: in Calabar, where 120 in of rain per year fall, the rainfall remains around 15 inches per month from June to September.
The rains cease on mid-September in the far north, in the course of October in the center, and they linger on until November in the far south. Along with the sun, the hot and dry weather returns in the center-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, 7,936 feet high, it's cool above 5,000 feet. Another area with some elevations is the Jos Plateau, in the center of the country, where the temperature is fairly mild: in Jos, the capital of the Plateau State, which is located at 3,900 feet, during the day the temperature goes from 82 °F in January, to 88/90 °F in March and April, to 75/77 °F in July and August.
The altitude enhances the instability during the rainy months, so that the rains are more abundant than in surrounding areas, going from 50 inches per year in the capital, to more than 60 inches in the rest of the plateau.
Average temperatures - Jos
Min (°F)576164666464636363636157
Max (°F)828690888481777581848482

In the far south-east of Nigeria, the Cross River National Park is an even more humid area, with hills and low mountains, covered by forests.
Zuma Rock

The sea is warm all year round, although care must be taken with the currents. The amount of sunshine, however, is never great on the coast, even in the dry months.
Sea temperature - Lagos
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 center and north, while at night it can get even cold in the far north, and sometimes the Harmattan can bring sandstorms and dust, 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 the last rains linger on. 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 fibers (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.