Tanzania lies just south of the equator and has two rainy seasons rather than one. Temperature varies less than rainfall, and altitude matters more than season: Arusha at 1400 m is cool at night year round while the coast is hot and humid throughout.
Dry season, June to October
The main safari season. Vegetation thins, animals concentrate at water, roads are firm and the sky is clear. In the Serengeti the herds are in the west and then the north, with the Mara crossings from around July to September. Days are warm, nights cool, and the northern parks are busy. This is also the best window for Kilimanjaro and for diving in the north of Zanzibar.
Short rains, November and December
Brief afternoon showers that rarely disrupt travel. The landscape greens, migrant birds arrive and the herds move back into the southern Serengeti. Prices are lower than in the peak and the parks are quieter. A good compromise season.
January and February
A dry gap between the two rainy seasons. This is calving season on the southern plains around Ndutu, with hundreds of thousands of wildebeest births in February and the predator activity that follows. It is also the best time on Mafia for whale sharks and one of the two good windows on Kilimanjaro.
Long rains, March to May
Heavy and sustained, especially April and May. Many southern camps close entirely, tracks become impassable, and Kilimanjaro routes are muddy and slippery. On the other hand prices fall by half, the landscape is spectacular and the northern parks remain accessible. Zanzibar is wet and many beach hotels close in April.
Zanzibar
The island is hot and humid year round, with a peak in the mid thirties from December to February. The long rains in April and May are the only period to avoid. July and August are drier and slightly cooler, and it is high season on the beaches even when the mainland is in low season.
Kilimanjaro
Two windows: January to mid March, which is warmer and clearer, and June to October, which is drier and busier. Avoid April, May and November. Full moon nights are popular for summit attempts and book out early.
Costs and crowds
July, August and the Christmas and New Year period are the most expensive and most booked. February is high season for the calving but less crowded than August. March, April, May and November are low season, with genuine savings and real weather risk.