Nigeria enjoys the humid tropical climate type. Because of its location just north of the equator, Nigeria enjoys a truly tropical climate characterised by the hot and wet conditions associated with the movement of the Inter-Tropical Convergence Zone (ITCZ) north and south of the equator.
The country experiences consistently high temperatures all year round. Since temperature varies only slightly, rainfall distribution, over space and time, becomes the single most important factor in differentiating the seasons and climatic regions. Rainfall occurrence and distribution are however dependent on the two air masses that prevail over the country. Their influences are directly linked to the movement of the ITCZ, north and south of the equator.
The two air masses are the Tropical maritime (Tm) and the Tropical continental (Tc). The former is associated with the moisture-laden south-west winds (south westerlies) which blow from the Atlantic Ocean, while the latter is associated with the dry and dusty north-east winds (easterlies) which blow from the Sahara Desert. When the ITCZ, the zone of convergence of the two air masses, is to the south of the equator, the north-east winds prevail over Nigeria, thus producing the dry-season conditions.
Conversely, with the movement of the ITCZ into the Northern Hemisphere, the rain-bearing south westerlies prevail as far inland as possible to bring rain fall during the wet season. The implication is that there is a prolonged rainy season in the far south, while the far north undergoes long dry periods annually. Nigeria, therefore, has two major seasons, the dry season and the wet season, the lengths of which vary from north to south.
The seasonal pattern of climatic conditions over Nigeria gives rise to four seasons in the south and two in the north. This is the result of annual total rainfall occurrence and distribution, which is more predominant in the south than in the north. The mean annual rainfall along the coast in the south- east is 4000mm while it is only 500mm in the north- east.
Southern Nigeria: The four observed seasons are:
(i) The Long Rainy Season: This starts in March and lasts to the end of July, with a peak period in June over most parts of southern Nigeria. It is a period of thick clouds and is excessively wet particularly in the Niger Delta and the coastal lowlands. It is marked by humidity with values hardly below 85 per cent in most parts of the forested south.
(ii) The Short Dry Season: This is experi- enced in August for 3-4 weeks. However, the real dry period known as the "August break" is generally observed in the last two weeks of August in most parts of southern " Nigeria".
(iii)The Short Rainy Season: This brief wet period follows the "August break" from early September to mid-October, with a peak period at the end of September. The rains are not usually as heavy as those in the long rainy season, although the spatial coverage over southern Nigeria is similar. The two periods of rainfall intensity give the double maxima phenomenon of the rainy season characteristic of southern Nigeria. The short dry season in August between these two rainy periods allows for harvesting and planting of fast-growing varieties of grains, such as maize.
(iv) The Long Dry Season: This period starts from late October and lasts to early March with peak dry conditions between early December and late February. The period witnesses the prevailing influences of the dry and dusty north-east winds, as well as the 'harmattan' conditions. Vegetation growth is generally hampered, grasses dry and leaves fall from deciduous trees due to reduced moisture.
Northern Nigeria: The climatic conditions in the northern part of Nigeria exhibit only two different seasons, namely, a short wet season and a prolonged dry season. Temperatures during the day remain constantly high while humidity is relatively low throughout the year, with little or no cloud cover. There are, however, wide diurnal ranges in temperature (between nights and days) particularly in the very hot months. The mean monthly temperatures during the day exceed 36°C while the mean monthly temperatures at night fall, at most times, to below 22°C.
(i) The Long Dry Season: This period extends from October to mid-May. With the ITCZ in the Southern Hemisphere, the north-east winds and their associated easterlies over the Sahara prevail over the country, bringing dry conditions. The harmattan period during December- January is more intense and longer in the north than in the south. This is the period of little or no cloud cover, resulting in wide diurnal ranges of temperature .
(ii) The Wet Season: The wet season covers a relatively short period, from June to September. Both the number of rain days and total annual rainfall decrease progressively from the south to the north. The rains are generally convectional, heavy and short in duration, often characterised by frequent storms. This results in flash floods, and in some places also in sheet or gully erosion.
The few high plateaus of Jos and Biu, and the Adamawa highlands, experience climatic conditions which are markedly different from the generalised dry and wet period in northern Nigeria. Temperatures are 5 - 10°C lower due to high altitude than in the surrounding areas. Similarly, the annual rainfall figures are higher than in areas around them, particularly on the windward side.
Nigeria can, thus, be broadly divided into the following climatic regions:
(a) the humid sub-equatorial, in the southern lowlands
(b) the hot tropical continental, in the farnorth
(c) the moderated sub-temperate in the high plateaus and mountains
(d) the hot, wet tropical, in the hinterland (the middle-belt)