|
Climate
|
Ghana's climate, like that of the rest of the Guinea Coast, is determined
largely by the interplay of two air masses: a hot, dry continental
air mass that forms over the Sahara and a warm, humid maritime tropical
air mass that forms over the South Atlantic. Both air masses move
toward the equator with their hemispheric winds and meet at the Guinea
Coast for several months each year. Continental air moves southward
with the northeast trade winds, known in Western Africa as the harmattan,
and maritime tropical air moves northward with the southwest trades.
The zone where these air masses converge is characterized by seasonal
line squall rainfall. The convergence zone itself oscillates north
and south, following the seasonal movements of the overhead sun and
the thermal equator; it reaches its most northerly position in the
central Sahara, about latitude 21° C, in August, and its most southerly
position about 7° C, a few miles north of the Ghana coastline, in
January. Rains occur when the dominant air mass is maritime tropical,
and drought prevails when continental air and the harmattan dominate.
In the savannah country north of the Kwahu Plateau, there are mainly
two seasons, a dry season from November to March, with hot days and
cool nights under clear skies, and a wet season that reaches its peak
in August and September. The mean annual rainfall is between 45 and
50 inches (1,145 and 1,270 millimetres), but there is a marked moisture
deficit because of the long, intensely dry season that follows. In
the southern forest country, where the annual mean rainfall from north
to south has a range of about 50 to 86 inches, there are two rainy
seasons-one from April to July and a lesser one from September to
November-and two relatively dry periods that occur during the harmattan
season, from December to February, and in August, which is a cool,
misty month along the coast. In the Accra Plains, anomalously low
annual mean rainfall figures vary from 40 inches to less than 30 inches,
and the rainfall variability and the vegetation bear close resemblance
to conditions in the northern savannah zone.
Temperatures show much more regional uniformity. The annual mean temperature
is from 78° to 84° F (26° to 29° C), and the daily range only some
10° to 15° F (6° to 8° C) along the coast, and some 13° to 30° F (7°
to 17° C) in the north. Average relative humidity range from nearly
100 percent in the south to 65 percent in the north, although during
the harmattan season figures as low as 12 percent have been recorded
in the north and around Accra. Enervating conditions produced locally
by the combination of high temperatures and high humidity is moderated
by altitude in the higher parts and by land and sea breezes along
the coast. In general, the hottest months are February and March,
just before the rains, and the lowest temperatures occur in January
or along the coast in August.
|
|
|
