Nigeria is about one-and-a-half times as large as the state of Texas and extends northwards to 14°N. It is bordered by Benin on the west, Niger on the north, and Chad and Cameroon on the east. To the south it has a coast on the Gulf of Guinea. An extensive low coastal plain rises gradually inland to hill and plateau country with an average height of 460-920 m/1,500-3,000 ft. Only in the southeast does Nigeria have mountains significantly higher than this.
The key to an understanding of the weather and climate in Nigeria and neighbouring countries farther west along the coast is the annual migration of the intertropical belt of cloud and associated heavy rain, high humidity, and relatively low temperature. Drier and sunnier weather, with higher temperatures, prevails on the northern side of this belt of cloud and rain. The belt of cloud and rain lies on the southern side of the point where the southwesterly to westerly winds of the Guinea monsoon give way to the northeast trade winds, or harmattan, which are dry and bring higher temperatures. The discontinuity between these winds, often called the intertropical convergence, lies over or near the coast in December and January and moves north to about 20°N by July and August. It then returns southwards rather more rapidly between September and December. Thus much of Nigeria and the region to the west experiences two rainy periods as the intertropical convergence moves north or south; but in the north the two rainy seasons merge to give a single wet season between July and September. This can be seen by looking at the tables for Kano in northern Nigeria. There is a single rainy season just after the time of high sun. (a similar pattern can be seen in the table for Tamale in northern Ghana.)
On the other hand, places on or near the coast have two rainy seasons with maximum rainfall in May or June and again in October. Although in the south near the coast no month is completely dry, there are two relatively drier periods between December and February and between July and September. In the north there is a single long dry season between October and April. At this time there is very little rain in the north and temperatures are warm to hot with a very low relative humidity. During this season the harmattan, which is often dust-laden, blows from the northeast day after day. During the period December to February the harmattan penetrates south so that the whole region, except a strip along the coast, is affected by it. For most of the year the coast has southwesterly winds; but on a few days these are overcome by the harmattan which brings its higher temperatures, lower humidity, and dusty air right to the coast. This brings some relief from the heat and humidity which prevail here for most of the year.
On the coast the period from December to February is least likely to experience rainy days, and this dry period is more clearly recognizable than the 'little dry season' between July and September. Inland, and particularly towards the north, the time of arrival of the rains and the amount of rain may vary from year to year.
The wettest parts of Nigeria are the coastal region of the Niger delta and the mountainous border with Cameroon in the southeast. Here the annual rainfall exceeds 2,500 mm/100 in, as compared with 1,250-1,500 mm/50-60 in in much of the west and centre of Nigeria. In the far north annual rainfall is below 1,000 mm/40 in almost everywhere and in places it is as low as 600 mm/24 in. Here the rainy season is rather short and the dry period is prolonged.
The table for Lagos is representative of the southern coast of Nigeria, that for Ibadan of the inland districts of the centre and west, and that for Kano of the dry northern region. Temperatures rise very high in the north during the period March to May before the arrival of the rains, but the rainy season may be equally unpleasant because of the higher humidity brought by the moist southwesterly winds. On the coast high humidity and constant high temperatures with very little relief make the weather rather uncomfortable throughout the year.
Hours of sunshine average from six hours a day during the rainy season to as many as ten in the dry season in the north of the country. Near the coast they average about three hours a day in the wettest months to six or seven hours during the driest period of the year.
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