An interesting phenomenon occurs beginning in December or January, known as the harmattan. That is a dry and dusty wind blowing south off of the Sahara desert into the Gulf of Guinea. The heavy amount of dust in the air can severely limit visibility and block the sun for several days, comparable to a heavy fog. At night time, in the beams of a car's headlights, you can actually see the dust in the air. It gets into and onto everything, even a closed house will become dusty inside as it seeps through the screens and under doors; outside you can see the fine yellow silt covering the cars. Naturally, it is a health hazard, as incidents of asthma and other breathing difficulties increase significantly during this period. Another less common malady is harmattan eye; the eye becomes red, oozes puss, and becomes swollen as a result of the minute particles getting under the eye lid. If you visit Ghana during this period, bring a good moisturizing skin lotion, moisturizing eye drops and don't forget lip balm.
you can visit http://www.ghanaweb.com/GhanaHomePage/geography/climate.php for a detailed report