~ Students may refer to one climatic type from either a tropical or temperate region. These notes apply to the TROPICAL CONTINENTAL (Savanna) Type Climate of West Africa.
~ Reference should be made to temperature, precipitation, winds and pressure change. It would be pleasing to see some supporting statistics in relation to temperature and precipitation changes, but if not students must use qualified descriptive statements.
As insolation is high, high temperatures prevail all the year
– 35 to 25 degrees centigrade
– this climate type is distinguished by a hot wet and a cooler dry season
– humidity highest in the wet season
– evaporation rates remain high during the cooler dry season. Rainfall occurrence is associated with the movement of the ITCZ zone towards the tropic in association with the apparent movement of the overhead sun’s position
– as this occurs low pressure prevails with moist in blowing winds and rising air currents leading to convection rainfall during the hot season
– amounts are most reliable towards the equatorial latitudes averaging 800 mms.
– becoming less reliable towards the hot desert margins averaging 300 to 400 mms. (In some years persistence of high pressure over the outer margins of the Savanna e.g. the Sahel zone prevents the ITCZ and in blowing moist winds from extending into these marginal areas leading to drought conditions. The variation from the normal seasonal weather patterns is partly linked to deviations in the path of Jet Streams in the higher atmosphere. Global warming is a further contributory factor. Refer back to this page
The cooler dry season in the savanna belt occurs as high pressure and dry out blowing winds prevail when the overhead sun and ITCZ move away to extend beyond the Equator towards the other tropics. Refer back to this page
There are
four seasons:
~ a dry and cool season,
Kaka,
(mid-November to February), marked by cool and dry weather plus
occasional dusty haze:
~ the dry and hot season,
Bazara
(March to mid-May) when temperatures climb up to 40*C and which is a transition period between the
harmattan (dry wind) and the wet season;
~ the wet and warm season,
Damina (mid-May to September), is the proper wet season
when the lowest diurnal temperature is recorded;
~ a dry warm season,
Rani (October to mid-November) marked by high
humidity and high temperature second to
Bazara in hotness.
Can you match this description to the climate graph above?
The mean annual rainfall ranges from over 1,000 mm in
the upstream Basement Complex area to approximately 400 mm in the middle
part of the basin and less than 300 mm near Lake Chad. However, climatic
variability has resulted in these mean annual rainfall values being
unrepresentative for different periods. Hess et al. (1995) calculated an
average decline in annual rainfall of 8 mm yr-1 between 1961-90 for the
north-eastern arid zone of Nigeria (i.e. the middle and lower part of
the basin). Since the mid-1990s the decreasing trend in annual rainfall
seems to have been reversed.
JAN 2010 Q.7 Account for the distinctive characteristics of one climatic type chosen from either a tropical or a temperate region. Theme 1.3 [25]