Porto Santo, where it's located

On the small island of Porto Santo, 43 kilometers (27 miles) north-east of Madeira, the climate is subtropical oceanic, with very mild winters and pleasantly warm summers.
Daytime temperatures hover around 18/19 °C (64/66 °F) from December to April, they are above 20 °C (68 °F) between May and November, and reach 25 °C (77 °F) in August and September. Nighttime temperatures hover around 13/15 °C (55/59 °F) from December to May and around 17/20 °C between June and October.
Here are the average temperatures recorded at the airport.
Porto Santo - Average temperatures
Min (°C)131313141517192020181614
Max (°C)181819192122242525232119
Min (°F)555555575963666868646157
Max (°F)646466667072757777737066

The prevailing winds, the northeast trade winds, bring a bit of cloudiness, humidity and rain on inland hills, while they keep pleasant the summer temperatures along the coasts. During winter, however, Atlantic disturbances can affect the island, preceded by southerly winds.
The rains are not abundant, since in the plain they do not reach 400 millimeters (16 inches) per year, and are concentrated in the period from October to mid-April, when they are relatively frequent, while in summer they are virtually absent.
Here is the average precipitation at the airport.
Porto Santo - Average precipitation

Porto Santo is a more arid island than Madeira, however, it is a little greener in the Northern inland area, where there are some hills: the highest peak is Pico do Facho, 517 meters (1,696 feet) high.
The amount of sunshine in Porto Santo is decent even in winter, when there is an alternation between sunny periods and bad weather periods, while it's good (but not great) in the summer months. In fact, even in midsummer cloud banks can form on the Atlantic Ocean, even if it does not rain.
In the period from June to September, Porto Santo can be affected by the Leste, a hot and dry wind from the Sahara, which can raise the temperature above 30 °C (86 °F) and bring a fine red powder. However, this happens a bit more rarely than in Madeira.

Porto Santo, panorama

The temperature of the sea in Porto Santo is not very high: it is about 18 °C (64 °F) in winter and spring, it's still very cool in June, around 20 °C (68 °F), and it reaches 23 °C (73 °F) in August and September, when it becomes warm enough for swimming.
Here are the average sea temperatures.
Porto Santo - Sea temperature
Temp (°C)181818181920212323222119
Temp (°F)646464646668707373727066

Best Time

Given that it has a very mild climate, you can visit Porto Santo all year round, but from mid-October to mid-April there may be rains and thunderstorms; the best time for swimming and sun bathing is the summer (July to September), while if you want to visit cities and go on excursions you can choose the spring, from mid-April to June.

What to pack in the suitcase

In winter: bring clothes for spring and autumn, a sweater, a jacket, a raincoat or umbrella.
In summer: bring light clothing, but also a scarf for the wind, a sweatshirt and a light jacket for the evening.