Kenya has complicated and rather unpredictable weather patterns, making it harder to decide the best time to visit, and the impact of climate change is striking hard. Broadly, the seasons are: hot and dry from January to March; hot and wet from April to June (the “long rains”); warm and dry from July to October; and warm and wet for a few weeks in November and early December – a period called the “short rains”. At high altitudes, it may rain at almost any time. Western Kenya, including the Maasai Mara, has a scattered rainfall pattern influenced by Lake Victoria, while the eastern half of the country, and especially the coast itself, are largely controlled by the Indian Ocean’s monsoon winds – the dry northeast monsoon (kaskazi) blowing in from November to March or April and the moist southeast monsoon (kusi) blowing in from May to October. The kusi normally brings the heaviest rains to the coast in May and June.
Temperatures are determined largely by altitude: you can reckon on a drop of 0.6°C for every 100m you climb from sea level. While the temperature at sea level in Mombasa rarely ever drops below 20°C, even just before dawn, Nairobi, up at 1660m, has a moderate climate, and in the cool season in July and August can drop to 5°C at night, even though daytime highs in the shade at that time of year easily exceed 21°C and the sun is scorching hot. Swimming pools are rarely heated, and only those on the coast are guaranteed to be warm.
The main tourist seasons tie in with the rainfall patterns: the biggest influxes of visitors are in December–January and July–August. Dry-season travel has a number of advantages, not least of which is the greater visibility of wildlife as animals are concentrated along the diminishing watercourses. July to September is probably the best period, overall, for game-viewing, with early September almost certain to coincide with the annual wildebeest migration in the Maasai Mara. October, November and March are the months with the clearest seas for snorkelling and diving. In the long rains, the mountain parks are occasionally closed, as the muddy tracks are undriveable. But the rainy seasons shouldn’t deter travel unduly: the rains usually come only in short afternoon or evening cloudbursts, and the landscape is strikingly green and fresh even if the skies may be cloudy. There are bonuses, too: fewer other tourists, reduced prices and often perfect light for photography.