Map from Google - Cape_Verde

In the Cape Verde Archipelago, the climate is very unusual, namely tropical arid, with temperatures generally not too high, despite the latitude, due to a cool sea current and to the trade winds, which steadily blow from the northeast, especially from November to May.
The islands are divided into two groups: in the north the Barlavento or Windward Islands (Santo Antão, São Vicente, Santa Luzia, São Nicolau, Ilha do Sal, Boa Vista), which have a more arid climate, and in the south the Sotavento or Leeward Islands (Maio, Santiago, Fogo, Brava), which are a bit more rainy. The Barlavento Islands to the north are definitely desert, since they receive less than 100 millimetres (4 inches) of rain per year, while the southern islands receive between 200 and 300 mm (8 and 12 in) of rain, so their climate is semi-desert.
In both groups of islands, in lowland areas or along the coasts there are rare rains concentrated in the period from August to October (and sometimes in July), which is also the warmest time of the year. The rains are quite irregular, which means that in certain years the rains are very scarce or absent, while in others there may be heavy rainfall, usually concentrated in a few days (or even just one), which may result in landslides and flooding, also because of the scarcity of vegetation, as occurred for example in September 2012 in Sal and Boa Vista.
In Mindelo, in the northern (Barlavento) island of São Vicente, only 97 mm (3.8 in) of rain per year fall, of which 45 mm (1.8 in) in September. Here is the average precipitation.
Average precipitation - Mindelo
Prec. (mm)00000002545205297

In the capital Praia, located in the island of Santiago, in the Sotavento Islands, therefore the rainiest, an average of 250 mm (10 in) of rain per year fall, of which 95 mm (3.7 in) in August and 115 mm (4.5 in) in September. Here is the average precipitation.
Average precipitation - Praia
Prec. (mm)2000005951153082257

As for the temperature, in Cape Verdes there is a hot and muggy period from July to November, and a more temperate and dry period from December to June, during which the north-east trade winds blow steadily. During the coolest season, from January to March, daytime temperatures are around 22/23 °C (72/73 °F) in the Barlavento islands, and around 25 °C (77 °F) in the Sotavento, while the night minimum is a few degrees lower, around 18 °C (64 °F) in the north and 19/20 °C (66/68 °F) in the south.
Here are the average temperatures of Mindelo, which as we said is located in the northern island of São Vicente.
Average temperatures - Mindelo
Min (°C)191919191921222323232221
Max (°C)232223242426272726262624
Min (°F)666666666670727373737270
Max (°F)737273757579818179797975

The southern islands are slightly warmer, as we can see from the temperatures of Praia, located in the southern (Sotavento) island of Santiago.
Average temperatures - Praia
Min (°C)201920212122242425242322
Max (°C)252526262728282929292826
Min (°F)686668707072757577757372
Max (°F)777779798182828484848279

In Cape Verde it's rarely hot, however, the temperature can sometimes reach 35/37 °C (95/99 °F) from May to October, when the wind blows from the desert.
Even during the dry period, from December to June, on the northern slopes of hills and mountains, a bit of cloudiness can form, and a bit of rain or drizzle can fall. In fact, Cape Verde is an archipelago made of volcanic islands, and in some of them we find mountains, among which Pico de Fogo, 2,829 metres (9,282 feet) high, in the island of Fogo, is the tallest: on the northern slopes of the island there is even a bit of greenery. On the island of Santiago, where the capital Praia is located, we find Pico da Antónia, 1,394 metres (4,573 feet) high, while in Santo Antao we find Topo da Coroa, 1,979 metres (6,493 feet), and Pico da Cruz, 1,585 metres (5,200 feet), and finally in São Nicolau we find Monte Gordo, 1,312 metres (4,304 feet).
The north-east trade winds begin to blow in November, and gradually lower the temperature; the wind attracts fans of windsurfing and kitesurfing especially in the eastern islands (Santa Maria, Boa Vista), from November to May. When weather conditions permit, the trade winds can be replaced by the Harmattan, a dry wind that blows from the Saharan desert, and can bring dust and sand storms, reducing visibility and hampering human activities.
The amount of sunshine in Cape Verde is high all year round, but it's slightly better between March and June. Between July and early October, ie the warmest and relatively rainy period, the sky may be cloudy for longer or shorter periods. In Praia, between July and September, the sunshine amount drops to 6/7 hours per day, while in the northern islands, less affected by cloud banks, it remains a little higher (7/8 hours).
Sunshine - Praia
Sun (hours)78910109767887

In the ocean south of Cape Verde, there is an area in which hurricanes, tropical cyclones of the Atlantic, start to form; after they are formed, they move towards the west or the northwest and head to the Caribbean. Some of these can brush the islands of Cape Verde, or they can directly hit them, especially the southern ones, although when they hit them they are usually still in the stage of tropical depression, and typically reach hurricane intensity when they're in the open ocean. However, the period when this happened in the past ranges from mid-August to mid-September or early October.

Cape Verde, Mindelo

The temperature of the sea in Cape Verde is high in summer, while it becomes a bit cool in winter, but still acceptable for swimming, as can be seen from the following table.
Sea temperature - Praia
Sea (°C)232323232324262727272625
Sea (°F)737373737375798181817977

In the northern islands, the sea is slightly cooler in the winter months, when it drops to 22 °C (72 °F).

When to go

Having a good climate, in theory, Cape Verde can be visited throughout the year. But it is necessary to make some clarifications. Lively breezes often blow, so that they can give a cool feeling in the winter months, especially in the days when the temperature is lower, and those who are sensitive to cold could need a jacket, moreover, the brisk wind can make the sea rough. This mainly occurs from January to March.
Although there is often little rain even in the period that is in theory the wettest, from August (and sometimes July) to October, during which it may not rain for weeks, occasionally there can be torrential rains, though short-lived, so this period can be avoided for safety. Besides, in this period the sky is often cloudy.
So all in all you can choose the months of May, June, the first half of July, and November.
At noon, the sun is directly overhead twice a year, and it will stay in the vicinity from May to September: care must be taken to sunburn, because cool breezes can prevent you from perceiving the intensity of the sun's rays.

What to pack

In winter: light clothing, but also light scarf for the wind, sweatshirt or sweater, light jacket for the evening. To climb mountains, sunscreen, hiking shoes, sweatshirt and jacket for the evening; warm jacket and hat for Pico de Fogo.
In summer: summer clothes, a light sweatshirt for the evening, comfortable shoes for excursions, possibly a light raincoat for August and September. To climb mountains, sunscreen, hiking shoes, sweatshirt and light jacket for Pico de Fogo.