Do you need to know, when is the best time to go to Guinea-Bissau? Then you should take a look here, what weather you can expect there during the year.
The climate of Guinea-Bissau is tropical. You can generally expect hot and humid weather. The coastal region of the country and most of the inland offer a tropical climate. There is monsoonal rainy season from June to November - with southwestern winds, the dry season is from December to May - with northeastern winds.
When speaking about the usual weather in this state, the information is relevant for example for these popular places: Bafatá, Bijagos Islands, Bissau, Bolama, Cacheu, Gabú, Varela.
Let's look now what weather you can expect in the area around the capital and in many other cities. The warmest month here is June, the average maximum temperature is 32.7 ℃ (90.86 ℉). The average minimum temperature in this month is 20.4 ℃ (68.72 ℉). The coldest month here is March, the average maximum temperature is 30.3 ℃ (86.54 ℉). The average minimum temperature in this month is 20.5 ℃ (68.9 ℉). The most rainy month here is March, the average number of rainy days is 24.3. The least rainy month here is June, the average number of rainy days is 1. 
Typical (usual) weather in Guinea-Bissau in January is this: The average highest temperature in Guinea-Bissau in January is 31.8 ℃ (89.24 ℉). The average lowest temperature is 19.4 ℃ (66.92 ℉). At the end of January you can expect lower temperatures, the average highest temperature is around 31.2 ℃ (88.16 ℉). The average number of rainy days in January is 12.2. The average rainfall is 349.5 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 11:26 (hours and minutes), in the middle of the month 11:30 and at the end of the month 11:36.These numbers above are valid primarily for the capital and the area around it.Typical (usual) weather in Guinea-Bissau in February is this: The average highest temperature in Guinea-Bissau in February is 30.6 ℃ (87.08 ℉). The average lowest temperature is 20.4 ℃ (68.72 ℉). At the beginning of February you can expect higher temperatures, the average highest temperature is around 31.2 ℃ (88.16 ℉). At the end of February you can expect lower temperatures, the average highest temperature is around 30.45 ℃ (86.81 ℉). The average number of rainy days in February is 17.3. The average rainfall is 269.5 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 11:36 (hours and minutes), in the middle of the month 11:44 and at the end of the month 11:54.These numbers above are valid primarily for the capital and the area around it.Typical (usual) weather in Guinea-Bissau in March is this: The average highest temperature in Guinea-Bissau in March is 30.3 ℃ (86.54 ℉). The average lowest temperature is 20.5 ℃ (68.9 ℉). At the beginning of March you can expect higher temperatures, the average highest temperature is around 30.45 ℃ (86.81 ℉). At the end of March you can expect lower temperatures, the average highest temperature is around 30.3 ℃ (86.54 ℉). The average number of rainy days in March is 24.3. The average rainfall is 638 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 11:54 (hours and minutes), in the middle of the month 12:04 and at the end of the month 12:13.These numbers above are valid primarily for the capital and the area around it.Typical (usual) weather in Guinea-Bissau in April is this: The average highest temperature in Guinea-Bissau in April is 30.3 ℃ (86.54 ℉). The average lowest temperature is 19.6 ℃ (67.28 ℉). At the beginning of April you can expect lower temperatures, the average highest temperature is around 30.3 ℃ (86.54 ℉). At the end of April you can expect higher temperatures, the average highest temperature is around 31.1 ℃ (87.98 ℉). The average number of rainy days in April is 21.5. The average rainfall is 414.2 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 12:13 (hours and minutes), in the middle of the month 12:23 and at the end of the month 12:32.These numbers above are valid primarily for the capital and the area around it.Typical (usual) weather in Guinea-Bissau in May is this: The average highest temperature in Guinea-Bissau in May is 31.9 ℃ (89.42 ℉). The average lowest temperature is 20.6 ℃ (69.08 ℉). At the beginning of May you can expect lower temperatures, the average highest temperature is around 31.1 ℃ (87.98 ℉). At the end of May you can expect higher temperatures, the average highest temperature is around 32.3 ℃ (90.14 ℉). The average number of rainy days in May is 11.3. The average rainfall is 82.7 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 12:32 (hours and minutes), in the middle of the month 12:40 and at the end of the month 12:45.These numbers above are valid primarily for the capital and the area around it.Typical (usual) weather in Guinea-Bissau in June is this: The average highest temperature in Guinea-Bissau in June is 32.7 ℃ (90.86 ℉). The average lowest temperature is 20.4 ℃ (68.72 ℉). At the beginning of June you can expect lower temperatures, the average highest temperature is around 32.3 ℃ (90.14 ℉). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 12:45 (hours and minutes), in the middle of the month 12:48 and at the end of the month 12:48.These numbers above are valid primarily for the capital and the area around it.