One of the first things you need to know about the weather in Ghana is that it is hot nearly every day, with daytime temperatures throughout the year approaching or topping 32 degrees Celsius/95 degrees Fahrenheit, and truly it doesn't get that much cooler in the evening.
In addition to being hot, during certain months of the year, it can get very humid. Accra experiences its hottest temperatures from November to April, with a brief reprieve in December through January when the humidity is reduced by the arrival of the harmattan or trade winds from the Sahara desert which brings with it lots of dust and fine silt. October through April is the dry season, followed by the rainy season, which runs from April to September, with the heaviest rainfall occurring in May and June. As with weather patterns elsewhere, the beginning, duration, and severity of these seasons vary from year to year.
Because of the tropical climate, it is important to bring hats, good quality sunglasses and enough sunscreen for daily application, insect repellant, swimsuits, and wash-and-wear clothing items (preferably cotton) appropriate for hot weather. A rash guard or swim shirts are recommended.
While very short skirts and shorts are generally not worn by women in Ghana, you may want to bring along a couple pairs of shorts for hanging around the house and for trips to the beach. Local beaches are filled with bikini clad sunbathers; even though in Accra, young women dress more liberally than in rural areas.
Conservative dress is necessary and appropriate in areas outside the capital city, especially, of course, in predominantly Muslim areas of the north. Another important point about the weather in Ghana is that during the rainy season it downpours daily, and often lasts for hours, so bring adequate rain gear such as umbrellas, lightweight raincoats, and waterproof shoes or boots if you're so inclined.
Men at work wear shirt and tie, and even jackets for meetings.