The country's warm, humid climate has an annual mean
temperature between 26°C and 29°C. Variations in the
elements of temperature, rainfall, and humidity that govern the
climate are influenced by the movement and interaction of the dry
tropical continental air mass, or the harmattan, which blows from
the northeast across the Sahara, and the opposing tropical maritime
or moist equatorial system. The cycle of the seasons follows the
apparent movement of the sun back and forth across the equator.
During summer in the northern hemisphere, a warm and moist
maritime air mass intensifies and pushes northward across the
country. A low-pressure belt, or intertropical front, in the air
mass brings warm air, rain, and prevailing winds from the
southwest. As the sun returns south across the equator, the dry,
dusty, tropical continental front, or harmattan, prevails.
Climatic conditions across the country are hardly uniform. The
Kwahu Plateau, which marks the northernmost extent of the forest
area, also serves as an important climatic divide. To its north,
two distinct seasons occur. The harmattan season with its dry, hot
days and relatively cool nights from November to late March or
April, is followed by a wet period that reaches its peak in late
August or September. To the south and southwest of the Kwahu
Plateau, where the annual mean rainfall from north to south ranges
from 1,250 millimeters 2,150 millimeters, four separate seasons
occur. Heavy rains fall from about April through late June. After
a relatively short dry period in August, another rainy season
begins in September and lasts through November, before the longer
harmattan season sets in to complete the cycle.
The extent of drought and rainfall varies across the country.
To the south of the Kwahu Plateau, the heaviest rains occur in the
Axim area in the southwest corner of Ghana. Farther to the north,
Kumasi receives an average annual rainfall of about 1,400
millimeters, while Tamale in the drier northern savanna receives
rainfall of 1,000 millimeters per year. From Takoradi eastward to
the Accra Plains, including the lower Volta region, rainfall
averages only 750 millimeters to 1,000 millimeters a year.
Temperatures are usually high at all times of the year
throughout the country. At higher elevations temperatures are more
comfortable. In the far north, temperature highs of 31°C are
common. The southern part of the country is characterized by
generally humid conditions. This is particularly so during the
night, when 95 to 100 percent humidity is possible. Humid
conditions also prevail the northern section of the country during
the rainy season. During the harmattan season, however, humidity
drops as low as 25 percent in the north.
Data as of November 1994