The country lies in the humid tropics within the tropical zone, between the equator and the Tropic of Cancer, and between the Atlantic Ocean and the Sudanese-Sahelian continental block. The country is subject to the movements and impacts of the Intertropical Convergence Zone (ITCZ), which migrates between the equator and the Tropic of Cancer over the course of the year broadly dictating its seasons. There are two pronounced seasons in Guinea-Bissau: the hot, rainy season, which lasts from May to November, and the hot, dry season from November to April. Temperatures range between 21.6°C and 39.3°C (monthly average: 30.5°C) in April and May, just before the rainy season, between 21.1°C and 31.5°C (monthly average: 26.3°C) in August and November and between 16.5°C and 38.5°C (monthly average: 27.7°C) in December to March. The climate of Guinea-Bissau is humid on the coastal central and southern territory (relative humidity between 62 and 87%) and drier in the rest of the territory (relative humidity between 58 and 68%). Precipitation does not vary greatly with elevation in Guinea-Bissau, although it does vary between coastal and inland areas; the coast displaying a tropical maritime condition receives some 60 to 120 inches (1,500 to 3,000 mm / year) of precipitation, whereas the interior is influenced by the tropical savanna climate, with greater variation in precipitation and temperature.
- Average yearly temperature has increased around 1°C.
- Observations suggest a shortening of the cool season during the months of December-February to only two months, December and January. Average annual temperature in the region of Bafata has increased by 0.3° C over 1961-2003.
- Observations of mean annual precipitation indicate that precipitation has decreased over the period of 1961-1990.
- Meteorological station data in eastern Guinea-Bissau indicate decreasing precipitation trends during the rainy season (May-October) and dry season (December-April), over the period of 1951-2000.
- A late start to the rainy season has been observed; now beginning around mid-June instead of early May. The dry season has extended in length, particularly in the eastern regions of the country.