Map from Google - Angola

Angola is a large African country, overlooking the Atlantic Ocean, and its climate is tempered, along the coast by a cold current, and in the inland areas by a plateau. The result is a sub-tropical climate almost everywhere, with a cool and dry season (called Cacimbo) from May to August, and a hot and rainy season that runs from mid-September to April in the north-east, from mid-October to April in the center, from November to March in the south, and from February to April in Luanda, while it's almost non-existent on the southern coastline.
The cool sea current which flows along the coast is called Benguela Current, and it makes the climate mild and dry, at least in the central and southern coast: in the southern coastal stretch 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 center and north, because of the greater proximity to the equator.
Climates in Angola
We have three main climate zones:
1 - In the northern area, where the plateau drops below 3,300 feet, and in the northern part of the Cabinda Enclave, 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 40 inches per annum everywhere.
Here is the average precipitation in M'banza-Kongo, located at 1,300 feet above sea level.
Average precipitation - M'banza-Kongo
Prec. (in)

The average temperature drops to around 70/72 °F from June to August, while it remains around 77/79 °F for a long period, that is from October to May. Very hot days are rare: usually the temperature does not exceed 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 west, where the sky is often cloudy even during the dry season, while in the east it becomes good, at least in the dry season. The best period to visit this area goes from June to August.
Here are the average temperatures of M'banza-Kongo.
Average temperatures - M'banza-Kongo
Min (°F)707070706863616164686870
Max (°F)848686868481777981848484

2 - Along the coast, the climate is desert in the south, semi-desert in the central area (between the two red lines, where lies the capital Luanda, where the only months with moderate rains are March and April), and semiarid in the north, where rainfall exceeds 20 inches per year. The three zones are separated by two red lines on the map. Although rainfall is little, fog, mists and low clouds are common especially in winter, ie from June to September in the south (see Namibe) and from July to October in Luanda. In the north, in Cabinda, sunshine is always lacking because winter fogs and mists are replaced by the rainy season cloud banks, 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. (in)

The average temperature is higher in the north: in Cabinda it goes from 79 °F from February to April, to 70 °F in July; in Luanda it goes from around 79 °F from February to April, to 68 °F in July and August.
Average temperatures - Luanda
Min (°F)737373737268646366707273
Max (°F)828486848177737375798182

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

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

The coastal area can experience sudden increases in temperature when winds blow 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 8,600 feet.
In the coastal area, the best time goes from May to October.
Iona National Park
3 - In the inland plateau, whose altitude generally ranges between 3,300 and 6,000 feet, the climate is sub-tropical, with a rainy season that runs from October to April in the north, and from December to March in the south. The rainfall generally goes from 47 to 63 inches per annum in the center and north (above the orange line on the map), while it drops below 40 inches in the south.
Here is the average precipitation in Huambo, located in the center of the plateau, at 5,600 feet above sea level.
Average precipitation - Huambo
Prec. (in)

Near the Equator, the sun passes its zenith at two fairly distant times of the year (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, so the wettest period goes from December to February, at the height of summer. During the cooler season, from May to August, on the plateau the sun shines and it never rains, but there are wide temperature variations 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 (°F)575757575246465055575757
Max (°F)777775777977778184817775

Between September and November, in the southern part near the border with Namibia, where the average altitude is around 3,300 feet, it can be very hot, with highs around 92/95 °F and peaks of 105 °F, as we can see from the temperatures of Ondjiva.
Average temperatures - Ondjiva
Min (°F)646463615245434655636464
Max (°F)908886868481818691959390

As mentioned, in the southern part of the rainfall is scarcer. Here the average rainfall in Ondjiva.
Average precipitation - Ondjiva
Prec. (in)

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

Kalandula Falls

The sea temperature in Luanda is warm for many months of the year, while it gets a little cool, but still acceptable for swimming, from July to September, when it drops to 72/73 °F.
Sea temperature - Luanda
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 64/66 °F.
Sea temperature - Namibe
Sea (°F)727377777368666466687072

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 with no rain. On the plateau, winter nights are cold, and the temperature can drop even below freezing, especially in the center and south. Even May and September are acceptable months: 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 first rains begin, and in May the last rains end.
In Luanda, you can swim 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 restrict 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 fibers, 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.