With reference to the Inter-Tropical Convergence Zone (ITCZ) and
the movement of air masses, describe and explain the varying
rainfall patterns in West Africa
The map shows several trends in rainfall as you travel south to north in West
Africa. Near to the coast, in a narrow band of land that always remains south of the
ITCZ (√), cities such as Lagos receive rainfall all year round with a two peak maxima
(√) – the May peak of 400mm being roughly twice the 200mm second peak in
October (√). Further north, rainfall decreases and distinct dry seasons become
apparent(√) . These become more prolonged further north so that the dry season
in Minna lasts only for December and January while that in Agades runs from
October through to April (√). Zinder, which lies between the above towns has a
November to March dry season. Rainfall totals also decrease the further north you
travel, from 1803mm per year in Lagos to 1328mm in Minna, 548mm in Zinder and a
desert-like 163mm in Agades (√).
The varying patterns of rainfall are a direct result of the relative position of
the ITCZ throughout the year. The ITCZ itself follows the thermal equator (√) –
the area of maximum heating of the earth’s surface by the sun (√) – which lies
between the tropics of cancer and capricorn. The ITCZ is a low-pressure area and a
zone of convergence (√) and is feed by winds originating from two main air masses.
To the south lies the warm, moist and unstable Tropical Maritime air mass (√),
originating over the Atlantic. This gives rise to southeasterly trade winds, deflected
by coriolis force as they cross the equator (√) to become the southwest monsoon
and bringing wet and warm weather (√). From the hot, dry and stable Tropical
Continental air mass (√) formed over the Sahara to the north comes the hot, dry
wind known as the Harmattan (√).
In January the ITCZ is found just north of Lagos on the west African coast
and the city is therefore influenced by rainfall from the wet and warm monsoon
winds blowing into the ITCZ (√). As the ITCZ moves north the rainfall intensifies
with the main band of thunderstorms around 400km to the south giving Lagos its
first rainfall peak of the year (√). Meanwhile, north of the ITCZ, in Minna, Zinder
and Agades, hot, dry northeast trade winds from the TC air mass bring very dry
weather (√) and the dry season to northern west Africa. As the ITCZ travels north
and passes over these towns, rainfall profiles increase to their August peaks (√). As
the ITCZ retreats back towards the west African coast, the dry season quickly
settles once more in Agades as it is affected by winds from the TC air mass (√).
Lagos meanwhile experiences a second peak in rainfall due to the southward
movement of the maximum rain belt travelling just to the south of the ITCZ (√).