Namibia receives between 10 mm of rain a year on the coast and about 600 mm in the north-east, all of it between roughly December and April. Everything about travel planning follows from that gradient.
Dry season, May to October
The main season. Skies are clear, roads are firm, and water is scarce enough that animals concentrate at waterholes, which makes Etosha reliable. May and June are pleasant, with days of 25 degrees and nights near freezing in the interior; July nights can drop below zero in the desert and at Sossusvlei, a fact that surprises people who packed for Africa. September and October warm up quickly and are the best wildlife months, with October reaching 35 degrees and above.
Green season, December to April
Rain falls as short, heavy afternoon thunderstorms rather than continuous drizzle, and rarely stops travel outright, though gravel roads can wash out and river crossings in the north-east become serious. The reward is dramatic: the desert turns green for a few weeks, the Etosha pan may hold water and flamingos, migrant birds arrive and antelope calve. Prices drop and lodges are empty. Photography is at its best because of the clouds; game viewing is at its worst because water is everywhere.
The coast
Swakopmund and Walvis Bay follow their own rules. The Benguela current keeps temperatures between 15 and 25 degrees year round, with fog on around 180 days. December and January are the Namibian holiday season on the coast and the towns fill with domestic visitors.
By interest
Wildlife: June to October, peaking in September. Landscape and photography: February to April for green desert and dramatic skies. Hiking, including the Fish River Canyon trail: May to September only. Birding: November to March, when migrants are present. Stargazing: any dry month, with the Milky Way core visible from about April to September.
Temperatures
Interior summer days reach 35 to 40 degrees, winter days 20 to 25 with nights around zero. The desert has the widest daily range in the country: 30 degrees of swing between afternoon and dawn is normal. Pack for both, in layers, whatever the month.
Peak periods
Namibian and South African school holidays in December and January fill coastal towns and Etosha camps. July and August are the European high season and lodges book out months ahead. Easter is busy. Everything else is quiet, which is much of the appeal.