The Swahili coast has two dry seasons and two rainy seasons. The long rains (masika) fall from late March through May; the short rains (vuli) come in November. The best months for travel are June through October — dry, cooler, and with the southeast monsoon (kusi) blowing steady offshore winds — and January through February, hot but dry.
The October to March northeast monsoon (kaskazi) brings warm seas and the best diving and snorkelling visibility. Avoid the long rains if at all possible: roads become impassable, dhow trips are cancelled, and the humidity is brutal. Coastal temperatures sit around 26-32°C year-round; nights are warm.