Map from Google - Angola

Angola is a large African country in the southern hemisphere, overlooking the Atlantic Ocean, and its climate is tempered, by a cool sea current along the coast, and by altitude in the plateau which is found in the interior. The result is a sub-tropical climate almost everywhere, with a cool and dry season from May to August (called Cacimbo), and a hot and rainy season, which runs from mid-September to April in the north-east, from mid-October to April in the centre, from November to March in the south, and from February to April in Luanda, while it's almost non-existent on the southern coastline (which, therefore, is desert).
The cool sea current that flows along the coast, the Benguela Current, makes the climate mild and dry, at least in the central and southern part; in the southernmost stretch of the coast the climate is even desert. In inland areas, the rains are generally more abundant than on the coast, but even here they are more abundant in the centre and north, because of the greater proximity to the equator.
Now, let's have a look at the three main climate zones.

1- The tropical zone

The tropical area

In the northern area, where the plateau drops below 1,000 metres (3,300 feet), and in the northern part of the enclave of Cabinda (a small province detached from the main territory and overlooking the sea), the climate is tropical, with a rainy season that runs from mid-October to mid-May in the west (see M'banza-Kongo) and from mid-September to April in the east (see Dundo). Rainfall exceeds 1,000 millimetres (40 inches) per annum everywhere.
Here is the average precipitation in M'banza-Kongo, located at 400 metres (1,300 feet) above sea level.
Average precipitation - M'banza-Kongo
Prec. (mm)1151251702501001019752001551205

The average temperature drops to around 21/22 °C (70/72 °F) in the austral winter, from June to August, while it remains around 25/26 °C (77/79 °F) for a long period, that is, from October to May. Very hot days are rare: usually the temperature does not exceed 34/35 °C (93/95 °F), but during the rainy season the air is moist, so the heat is unpleasant. The sunshine amount is never good in the western part, where the sky is often cloudy even during the dry season, as can be seen from the sunshine hours in M'banza-Kongo.
Sunshine - M'banza-Kongo
Sun (hours)566666544454

Instead, in the eastern part the amount of sunshine becomes good, at least in the dry season.
Here are the average temperatures of M'banza-Kongo.
Average temperatures - M'banza-Kongo
Min (°C)212121212017161618202021
Max (°C)293030302927252627292929
Min (°F)707070706863616164686870
Max (°F)848686868481777981848484

The best period to visit this area goes from June to August, being the coolest, and within the dry season.

2- The coast

Climate of the coast

Along the coast (which also includes the southern part of the province of Cabinda), the climate is desert but mild in the south, semi-desert in the central area (between the two red lines, where lies the capital Luanda, in which the only months with moderate rains are March and April), and semiarid in the north, where rainfall exceeds 500 mm (20 in) per year. The three zones are separated by two red lines on the map. Although it rains little, fog, mists and low clouds are common, especially in winter, ie from June to October in the south (see Namibe), and from July to October in Luanda. In the north, in Cabinda, the sunshine is poor, because fogs and mists in winter are replaced by cloud banks in the rainy season, which here begins to be substantial and runs from late October to early May.
Here is the average precipitation in Luanda.
Average precipitation - Luanda
Prec. (mm)253510012520001263525374

The average temperature is higher in the northern part of the coast, which is closer to the equator: in Cabinda it goes from 26 °C (79 °F) from February to April, to 21 °C (70 °F) in July; in Luanda it goes from around 26 °C (79 °F) from February to April, to 20 °C (68 °F) in July and August. Here are the average temperatures of Luanda.
Average temperatures - Luanda
Min (°C)232323232220181719212223
Max (°C)282930292725232324262728
Min (°F)737373737268646366707273
Max (°F)828486848177737375798182

In Luanda, the sun does not shine very often, however the sunniest months are the dryest months of the first part of the year (January, February and May), the least sunny ones are those in which the cool current prevails (from July to October), and fogs and low clouds are more easily formed.
Sunshine - Luanda
Sun (hours)777677555566

The southern part of the coast is cooler: in Namibe (formerly Moçâmedes), the mean temperature goes from 24 °C (75 °F) in February and March, to 16.5 °C (61.5 °F) in July.
Average temperatures - Namibe
Min (°C)182020181513131315161717
Max (°C)262828272522202122232526
Min (°F)646868645955555559616363
Max (°F)798282817772687072737779

In the southern part of the coast the rains are very scarce, given that they drop below 50 mm (2 in) per year. Here is the average precipitation in Namibe.
Average precipitation - Namibe
Prec. (mm)6916100000012246

In the southern part of the coast, the sun shines more often in March and April (since it almost never rains), while in winter the sun becomes even rarer (since the cold current is felt more), especially from July to September.
Sunshine - Namibe
Sun (hours)778875344577

In the southern part of the coast, the best period is summer, since it is not too hot and it is sunny, while winter is mild, but damp and foggy.
Anyway, the coastal area can experience sudden increases in temperature when the wind blows from the interior, especially where the escarpment that separates the plain from the plateau is higher. In the central area, we find the highest peak of the country, Morro de Moco, with its 2,620 metres (8,600 feet).

Iona National Park

The sea temperature in Luanda is warm for many months of the year, while it gets a little cool, but all in all it's still acceptable for swimming, from July to September, when it drops to 22/23 °C (72/73 °F).
Sea temperature - Luanda
Sea (°C)272828282724232223252627
Sea (°F)818282828175737273777981

In the southern part of the coast (see Namibe), the sea temperature is lower, and it becomes very cool in the winter months, around 18/19 °C (64/66 °F).
Sea temperature - Namibe
Sea (°C)222325252320191819202122
Sea (°F)727377777368666466687072

3- The plateau

Highlands climate

In the inland plateau, whose altitude generally ranges between 1,000 and 1,800 metres (3,300 and 6,000 feet), the climate is sub-tropical, with a rainy season, which runs from October to April in the north, and from December to March in the south. The rainfall generally goes from 1,200 to 1,600 mm (47 to 63 in) per annum in the centre and north (above the orange line on the map), while it drops below 1,000 mm (40 in) in the south.
Here is the average precipitation in Huambo, located in the centre of the plateau, at 1,700 metres (5,600 feet) above sea level.
Average precipitation - Huambo
Prec. (mm)21015523015520002201252052501365

Near the equator, the sun passes its zenith in two moments of the year, quite separated from each other (at the equator this event takes place on March 21 and September 23), and this affects the rainfall pattern, producing two maxima within the rainy season, that is, in November-December and March-April in the north. On the contrary, in the south the two solar peaks are very close to each other, so the wettest period goes from December to February, at the height of the summer. During the coolest season, from May to August, on the plateau the sun shines and it never rains, but there are wide variations in temperature between night and day, so that nights are cold, with possible frost, while during the day it can be even warm.
Here are the mean temperatures in Huambo.
Average temperatures - Huambo
Min (°C)1414141411881013141414
Max (°C)252524252625252729272524
Min (°F)575757575246465055575757
Max (°F)777775777977778184817775

In Huambo the sun shines often in the dry period, while it does not shine very often in the rainy season, from October to April.
Sunshine - Huambo
Sun (hours)555689987544

In the southern part of the country, near the border with Namibia, where the average altitude is around a thousand metres (3,300 feet), between September and November, that is, before the rainy season, it can be hot, with highs around 33/35 °C (92/95 °F), and peaks of 40 °C (104 °F), as we can see from the temperatures of Ondjiva, at 1,100 meters (3,600 feet) of altitude.
Average temperatures - Ondjiva
Min (°C)181817161176813171818
Max (°C)323130302927273033353432
Min (°F)646463615245434655636464
Max (°F)908886868481818691959390

As mentioned, in the southern part of the plateau, rainfall is scarcer than in the central and northern part, and also the rainy season is shorter. Here is the average rainfall in Ondjiva.
Average precipitation - Ondjiva
Prec. (mm)10514512055300011550125620

In the southern part of the plateau, the sun shines even more often, especially in the summer, which here is less rainy.
Sunshine - Ondjiva
Sun (hours)78891011101011898

In the plateau area, the best time goes from May to September.

Kalandula Falls

When to go

The best time to visit Angola in its entirety runs from mid-May to August, being the coolest and sunniest period, at least in inland areas, while along the coast fogs and low clouds are frequent, but at least it doesn't rain. On the plateau, winter nights are cold, and the temperature can drop even below freezing, especially in the centre and south. May and September can be acceptable months as well: they are a bit warmer, which means that the plateau is less cold at night, but during the day it can get hot in the extreme south, while in the north-east in September the rains begin, and in May the last rains linger on.
For swimming and sunbathing, in Luanda you can go from December to June, when the sun often shines and it seldom rains, maybe avoiding March and April, which are the only relatively rainy months; the best month is probably May. Further south, it rains still less, but the sea is cooler, so you can limit the period to March and April. North of Luanda, the sunshine is increasingly scarcer and rainfall more abundant, so this coastal area it's not the best for beach tourism.

What to pack

In winter (June to August): for the coast, spring/autumn clothes, a sweatshirt or sweater and a jacket, especially south of Luanda; for the northernmost area, light clothes for the day, a sweatshirt and a light jacket for the evening, sun hat. For the south-central plateau, light clothes for the day, but also sweater, jacket and hat for the evening; hiking shoes.

In summer (December to February): along the coast, light clothing, a light sweatshirt for the evening, a light jacket for the southern part (see Namibe); for the northernmost area, light and loose-fitting clothes, of natural fibres, sun hat, a light sweatshirt for the evening, light raincoat or umbrella. For the south-central plateau, light clothes for the day, sun hat, a sweatshirt and a spring jacket for the evening, light raincoat or umbrella, hiking shoes.