Weather, Climate and Vegetation Belts

What you need to know

• Weather is the state of the atmosphere at a place at a particular moment, while climate is the average weather of a place worked out over at least 30 years. Weather changes hour by hour; climate is the long-run pattern.
• The elements of weather are temperature, rainfall, atmospheric pressure, humidity, wind speed and direction, cloud cover and sunshine duration. Each one has a standard instrument and a standard unit, and WAEC asks for instrument and unit together.
• Temperature is measured with maximum and minimum thermometers, often combined in Six's thermometer, in degrees Celsius; rainfall with a rain gauge, in millimetres; pressure with a mercury or aneroid barometer, in millibars; humidity with a wet and dry bulb hygrometer, as a percentage; wind direction with a wind vane; wind speed with a cup anemometer, in kilometres per hour or knots; and sunshine with a Campbell-Stokes sunshine recorder, in hours.
• Thermometers and hygrometers are housed in a Stevenson screen: a white, louvred wooden box standing about 1.2 m above the ground on legs, with the door facing north in Nigeria so that the observer does not let direct sunlight fall on the instruments. White paint reflects radiation and the louvres let air circulate freely.
• The factors controlling temperature are latitude and the angle of the sun's rays, altitude, distance from the sea, ocean currents, cloud cover, prevailing winds, and the nature of the surface. Temperature falls with height at a lapse rate of about 0.6 degrees Celsius for every 100 m, which is why Jos and Obudu are cool while Lokoja is hot.
• Nigeria's weather is governed by two air masses separated by the Inter-Tropical Discontinuity. The tropical maritime air mass is moist and comes from the Atlantic on the south-west monsoon; the tropical continental air mass is hot, dry and dusty and comes from the Sahara on the north-east trade wind, giving the harmattan from about November to February.
• As the ITD swings north between March and August, the rain belt follows it, so the rainy season starts earlier and lasts longer in the south than in the north. Coastal towns such as Warri and Calabar receive rain over nine months and more than 3,000 mm a year, while Maiduguri and Nguru receive rain for barely four months and under 700 mm.
• Southern Nigeria shows a double rainfall maximum, peaking around June and September with a short dry spell in August known as the August break or little dry season. Northern Nigeria shows a single maximum peaking in August.
• Rainfall is classified by origin as convectional, relief or orographic, and cyclonic or frontal. Convectional rainfall dominates in Nigeria: intense heating makes moist air rise, cool, condense and fall as heavy afternoon downpours with thunder, usually short and localised.
• Vegetation belts run roughly east to west in bands and change with rainfall as you go north. From the coast inland the order is saltwater or mangrove swamp forest, freshwater swamp forest, tropical rainforest, derived savanna, Guinea savanna, Sudan savanna and Sahel savanna, with montane vegetation on the Jos, Mambilla and Obudu highlands.
• Mangrove swamp forest grows in brackish tidal water in the Niger Delta and along the Lagos and Calabar creeks, dominated by red mangrove and white mangrove on stilt roots, with breathing roots called pneumatophores. It supplies fuelwood, fish and shellfish, and shelters the creeks from erosion.
• Tropical rainforest in Ondo, Edo, Cross River and Ogun has tall trees in three canopy layers, buttress roots, lianas, epiphytes and evergreen broad leaves because rainfall exceeds 1,500 mm and is spread over most of the year. Iroko, mahogany, obeche and opepe come from this belt and feed the timber and furniture trade.
• Guinea savanna, the widest belt in Nigeria, has tall grasses up to 3 m with scattered fire-resistant and deciduous trees such as locust bean, shea and isoberlinia. Sudan savanna has shorter grass and thorny acacia and baobab, and supports most of the country's groundnut, cotton, sorghum and cattle. Sahel savanna in the far north-east has sparse short grass, stunted thorn bushes and bare patches, and is the belt under greatest threat from desertification.
• Human activity has reshaped the natural vegetation: bush burning, shifting cultivation, overgrazing, fuelwood cutting and logging have turned much of the forest edge into derived savanna and are pushing desertification southward in Borno, Yobe, Sokoto and Katsina. Counter-measures include shelter belts, afforestation and the Great Green Wall programme.