The dry season from November to February is the best time to visit. Days are warm and clear, nights on the plateau are pleasantly cool (sometimes cold enough for a sweater), and the harmattan — the dusty wind off the Sahara — fills the air. The roads to the falls and the outlying landmarks are easy to drive.
The rainy season from May to September is when the plateau is at its greenest and the waterfalls are at their fullest, but it can be wet for days on end and some of the rougher roads turn to mud. March and April are usually the hottest months even on the plateau; April is the practical end of the cool, dry season.
Before any trip, check the current security situation in Plateau State. Jos has experienced periods of inter-communal violence over the past two decades; many visits pass without incident but conditions can change quickly. Stick to recommended routes and times, and travel with a knowledgeable local driver for any trips outside the city.