Cape Verde has a tropical climate with sunshine all year round. The northeasterly wind ensures pleasant temperatures for a lot of the year. The coolest and the breeziest months are from December to March. The 'rainy season' is in August and September but the rain may only be for a couple of days and some islands may not get any rain at all. The rainiest islands are the ones with mountains (Santiago, Santo Antao, Fogo, Sao Nicolau) and the ones with least rain are Maio, Sal & Boa Vista, where the majority of holiday makers visit. Taking Sal as an example, temperatures may range from 18-28 degrees C peaking in August/September. Temperature usually drops a few degrees in the evening and in winter a light sweater may be needed.
Daily hours of sunshine average from seven to ten and are highest in the months February to June. There is more cloud during the rainier months.
A couple of weather charts may be found at these links
Any time of year is good for a visit, although the peak of summer is very hot there is usually a good breeze to keep you cool. Even in the rainy season the rain does not usually last too long and although it does bring out the flies and mosquitoes, it turns the islands amazingly green and lush! Windy season is great for kite and windsurfing but on some beaches you may get sandblasted.