In Guinea Bissau, the climate is tropical, hot all year round, with a dry season from November to May, and a rainy season from June to October due to the African monsoon. The hottest period runs from February to May, before the rains, while the rainy season, though less hot, it's muggy. The heat is intense especially in the interior, while along the coast, where Bissau is located, it's a bit cooler because of the breeze.
Here are the average temperatures in Bissau, the capital.
Bissau - Average temperatures
Min (°C)192022232424232323232321
Max (°C)313233333231292829313131
Min (°F)666872737575737373737370
Max (°F)889091919088848284888888

The monsoon rains are very heavy, especially from July to September, when they can cause widespread flooding. In addition, in this period the sky is often cloudy. August is the rainiest month, and if possible it should be avoided. The rainfall is more abundant in the south, where it exceeds 2,000 millimeters (79 inches) per year, and less abundant in the north, where it amounts to around 1,500 mm (60 in) per year.
In Bissau, precipitation amounts to 2,000 millimeters (79 inches) per year. Here is the average precipitation.
Bissau - Average precipitation

In Bissau, the sun often shines from November to May, while in the rainy season the sky is often cloudy, especially in the wettest period, from July to September.
Bissau - Sunshine

During winter, the weather is sunny, and the air is dry because of the prevailing wind, the Harmattan, which blows from the desert, and can bring dust, making the sky whitish. Normally, the daytime temperature is about 30 °C (86 °F), but sometimes it can reach 40 °C (104 °F). However, this is the best time of the year. Starting from February, both temperature and humidity gradually increase, reaching a peak between March and May, when it can easily surpass 40 °C (104 °F). Starting from June, the rains lower the temperature to about 28/30 °C (82/86 °F), but moisture makes the heat muggy. However, at least on the coast the heat is tempered by the breeze.
Here are the average temperatures of Gabù, located in the north-east, where the heat is intense in the months preceding the monsoon.
Gabù - Average temperatures
Min (°C)161921232423222222222016
Max (°C)323537383733302930323331
Min (°F)616670737573727272726861
Max (°F)9095991009991868486909188

In Gabù, 1,400 mm (56 in) of rain fall per year.
Gabù - Average precipitation

In the rainy season, the sun in the north-east shines more often than on the coast.
Gabù - Sunshine

Guinea Bissau

The sea is always warm enough for swimming, although it is warmer from June to November, when the water temperature is about 28 °C (82 °F), while in March and April it becomes almost cool, since it drops to 23 °C (73 °F).
Bissau - Sea temperature
Temp (°C)252423232628282828282827
Temp (°F)777573737982828282828281

Tropical cyclones

In the ocean to the west of Senegal, there is an area in which hurricanes, the tropical cyclones of the Atlantic, start to form; after they are formed, they move towards the west or the northwest and head to the Caribbean. In general, the country is not affected by hurricanes because they develop at a considerable distance, however, occasionally a hurricane can be formed a short distance from the coast and affect Guinea Bissau as well, with rainfall, and also storm surges that can damage the coast, as happened at the beginning of September 2015 with Hurricane Fred.
Hurricanes are generally formed in this area in August and September.

Best Time

The best time to visit Guinea Bissau is winter, in particular the months of December and January, which are the least hot, among those in which it does not rain. It may be useful to bring a scarf, for the dust brought by the wind. For a beach holiday, the dry season, from November to May, is preferable.

What to pack

In winter: in Bissau and the coast, bring light clothes for the day, a sweatshirt for the evening, a scarf for the wind-borne sand and dust. In inland areas, you can add a light jacket for the evening.
In summer: bring light clothing of natural fibers, a light sweatshirt and a light raincoat for the rain.