The coastal belt of Guinea and most of the inland region have a tropical climate, with a long rainy season lasting from April to November. The capital, Conakry, has a year-round average high of 84 °F (29 °C), and a low of 73 °F (23 °C), which shows that the country does not have an extreme range of temperatures.
The best time of year to visit Guinea is between November and March when the weather is moderate and the temperatures comfortable. From December to February the famous harmattan winds spread Saharan sand through much of the country but this is not a serious hinderance to travellers. The rainy season - which is very rainy indeed - is at its most extreme in July and August and this period is bad for travel not just because of the monsoon-like rain but also because many of the roads close as a result. The months of May, June, September and October are also good for travel as rain during these months is more moderate.
It should be noted that Guinea incorporates diverse landscapes and the climate can change quite substantially according to region: the mountainous areas are much cooler than the flatlands and savannah, for example. Plan your trip according to the specific weather of the area which you want to explore and your desired activities.