The country's warm, humid climate has an annual mean temperature between 26°C and 29°C. Variations in the principal 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.
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