Kenya straddles the equator and has two rainy seasons, but the country is so varied in altitude that generalisation is difficult. Nairobi at 1795 m is temperate; Mombasa at sea level is tropical; the northern deserts are hot in every month.
The dry seasons
The main dry period runs from late June to October and is the classic safari window: short grass, thin bush, animals at water, and the migration in the Masai Mara from July. A shorter dry spell from January to early March is drier still in some regions, is excellent for game and has fewer visitors than August.
The long rains, March to May
The heaviest rain of the year, especially in April. Roads in the parks become difficult, some camps close, and the coast is wet. In exchange, prices drop by up to half, the landscape is green and the birding is at its best with migrants present. April and May are the least reliable months for a first visit.
The short rains, November and December
Brief afternoon showers rather than sustained rain, and rarely enough to disrupt a safari. The parks are green, the calving season is beginning, and rates sit between low and high season. Late December is busy again for Christmas.
The migration
The herds cross into the Mara from around July and stay until late October, with crossings possible at any point in that window and none guaranteed. August and September are the peak and the most expensive. Note that the resident wildlife of the Mara is excellent without the migration and far cheaper in February.
The coast
Hot and humid year round, between 26 and 33 degrees. December to March is the driest and hottest and the peak season; July to October is cooler with the kusi wind; April and May are wet and many hotels close. Humidity is highest in March.
Mount Kenya and highlands
January and February, and August to September, are the climbing windows. The highlands are cool throughout: Nairobi nights drop to 10 to 12 degrees in July, which catches people out.
Costs and crowds
July to September and the Christmas period carry peak rates and require booking months ahead. April, May and November are low season with real savings. February is the quiet high-quality month that experienced visitors choose.
Two things people underestimate
The equatorial sun burns through cloud at 1800 m, and the open roof of a safari vehicle offers no shade at all — a hat and sunscreen matter more than the temperature suggests. And whatever the month, the vehicle leaves before six in the morning, when even the Mara is close to ten degrees.