Map from Google - Namibia

Namibia, the former South-West Africa, has a sub-tropical climate, desert along the coast and in the south, and arid with a rainy season from November to March in the north-central and north-eastern inland areas.
The country is crossed by the Tropic of Capricorn, but along the coast a cold current flows, called Benguela Current, while the interior is occupied by a plateau. It follows that the climate is cooler and drier than you might think. Nevertheless, in summer it can get very hot, especially at some distance from the sea and in areas located at not-too-high altitudes. There may be blistering days from September to March in the north, from November to January in Windhoek, which is located at a pretty high altitude, and from September to May in the center-south. On the contrary, during winter, the plateau can get cold at night, with possible light frosts at night.
The country is home to nature reserves and parks of great beauty, like the Etosha National Park.
Being in the Southern Hemisphere, its seasons are reversed from Europe or North America.

The driest region is the coastal area, where the Namib Desert is located, as well as the south, where there is the Kalahari Desert, which is slightly rainier, so that it's home to xerophyte plants like cacti and other succulent species. The rainiest area of Namibia is the north-east, where rainfall ranges from 20 to 24 inches per year. Winter is dry everywhere, while in non-desert regions (north and east) it rains in summer, from November to March, usually in the form of showers or thunderstorms in the afternoon. The peak of the rainy season is from January to March.

Along the coast, the climate is desert but mild: it almost never rains but the air is humid and there are often fogs, mists and low clouds, especially in the early hours of the day. The temperatures are spring-like all year round: the maximum ranging from 59/63 °F in winter to 68/72 °F in summer.
Here are the average temperatures of Swakopmund, in the central part of the coast.
Average temperatures - Swakopmund
Min (°F)596159555252484850525557
Max (°F)687068646468646161616466

The sea temperature is very cool, if not cold, all year round, however, it reaches 63 °F from January to March. Only in the northernmost part of the Skeleton Coast, the sea becomes a bit warmer, so that it reaches 71/73 °F in March.
Sea temperature - Swakopmund
Sea (°F)636363615957575757575961

As you can see from the average rainfall, in Swakopmund it hardly ever rains.
Average precipitation - Swakopmund
Prec. (in)

Along the coast, the temperature can suddenly increase, when the wind called Oosweer, which can also cause sandstorms, blows from inland. In addition, it's sufficient to move inland for not too many miles, to note the shrinking of the influence of the sea, so that the air temperature will increase rapidly, at least during the day and in summer. To understand how strong is the cooling influence of the sea, we can recall that in summer the temperatures in inland areas are higher than on the coast, even at altitudes above 5,000 feet. The coast is not recommended for beach tourism, but rather for watching birds, or animals such as sea lions at Cape Cross, and also the remains of shipwrecks of past centuries in the Skeleton Coast to the north. In the area of Sossusvlei in the west, there are huge orange sand dunes, probably the highest sand dunes in the world.

Sand Dunes in Sossusvlei

The inland areas are occupied by a plateau, whose altitude ranges from 3,900 to 5,600 feet. Occasionally, there are higher chains, and peaks exceeding 6,500 feet. In the southern regions (Hardap and Karas), in the warmer months (December and January), the maximum temperatures are around 95 °F at 3,300 feet of altitude. In the north, where the rainy season is more pronounced, in the warmer months the temperatures are a bit lower, with highs around 86 °F. North of the Tropic, the warmest period is from September to November, before the rainy season. For example in Tsumeb, the hottest month is October, when daytime temperatures are slightly higher than in January: 91 °F instead of 88 °F, while in Kasane, in the far north-east, the average maximum of October is 95 °F. Here are the average temperatures in Tsumeb.
Average temperatures - Tsumeb
Min (°F)646463595246465059646464
Max (°F)888684827975758188919088

In Tsumeb, 22 in or rain per year fall. Here is the average precipitation.
Average precipitation - Tsumeb
Prec. (in)

As mentioned, the north, where Tsumeb is located, is the wettest area of the country. In the vast Etosha National Park, where there are protected species such as the black rhino, we find the Etosha Pan, a salt lake which is dry most of the year, but during the rainy season becomes a shallow lake. In the wettest years, there may be flooding in the Etosha region and in the Cuvelai Basin.
The northeast is the rainiest area: the Kavango Region and the Caprivi Strip are covered by the savannah biome, also there are major rivers such as the Okavango and Zambezi, along whose banks the forest flourishes.

Elephants in the Caprivi Strip

In the capital Windhoek, in the center of the country and at 5,600 feet above sea level, the average temperature goes from 57 °F in June and July, to 75 °F in December and January. So the summer is hot despite the altitude, and much warmer than along the coast. In winter the days are very mild, indeed even slightly warm, but at night it can get cold. Here are the average temperatures.
Average temperatures - Windhoek
Min (°F)646361555045454854596163
Max (°F)868481797370707582848688

Winter is a very sunny season, during which it almost never rains. From November to April instead some rain falls, but only from January to March the rains reach on average 2.8/3.1 inches per month. The total annual precipitation of 14.6 in makes the climate arid, but in some years, when tropical air masses from Angola arrive up to here, in the rainy season it can rain a lot. Here is the average precipitation.
Average precipitation - Windhoek
Prec. (in)333.11.60.200000.51.41.814.6

In the southern part of the plateau, the climate is more arid, in fact as mentioned we are in the Kalahari desert. Here the summer rains are too poor to lower the temperature, so that the hottest season is just the summer, with highs of 93/95 °F from December to February. On the other hand, the southern position makes the winter colder, at the same altitude. Here are the average temperatures of Keetmanshoop, which is located at 3,300 feet of altitude.
Average temperatures - Keetmanshoop
Min (°F)646461554843414548545963
Max (°F)959390847770707579869093

The rainfall amounts to 6 inches per year, with a peak of 1.6 inches in February, so it is at a desert level.
Average precipitation - Keetmanshoop
Prec. (in)

When to go

The best time to visit Namibia as a whole is the austral winter, from June to August: it's a dry and sunny period, and it's the only one that allows to avoid the intense heat; on the plateau, however, nights are cold.
The coast, having a mild climate can be visited all year round, but in winter it's cooler, especially in August, while it's more mild, stable and sunny from November to February: hence, on the coast the best time is reversed compared with the rest of the country.
In the northeast, it's better to avoid the summer, from November to March, because of both the heat and the rains, but also September and October which may have torrid days.
Wanting to visit Namibia during the rainy season, for example at Christmas, you can choose the coast and the southern regions, bearing in mind that in inland areas and in particular in the Kalahari, the days are definitely hot, but at least it doesn't rain, or at most there may be some brief downpours or thunderstorms.

What to pack

In winter (June to August): on the coast, spring/autumn clothes, a sweatshirt or sweater and a jacket. In Windhoek and the plateau, spring/autumn clothes, with the possibility of adding or removing the outer layer, sweater, hat and warm jacket for the evening.

In summer (December to February): along the coast, spring/autumn clothes, t-shirt, shirt, sweatshirt and light jacket. In Windhoek and the plateau, light clothes for the day, sun hat, a sweatshirt for the evening, light raincoat or umbrella.