Map from Google - Porto_Santo

In the small island of Porto Santo, 43 km (27 mi) 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, while 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 17/20 °C between June and October.
Here are the average temperatures recorded at the airport.
Average temperatures - Porto Santo
Porto SantoJanFebMarAprMayJunJulAugSepOctNovDec
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 make pleasant the summer temperatures along the coasts. During winter, however, Atlantic disturbances can pass, preceded by southerly winds.
The rains are not abundant, since in the plain they do not reach 400 mm (16 in) 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.
Average precipitation - Porto Santo
Porto SantoJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec. (mm)504035251573425405070364

Porto Santo is a more arid island than Madeira, and is a little greener only in the inland northern area, where there are some hills: the highest peak is Pico do Facho, 517 metres (1,696 feet) high.
The amount of sunshine in Porto Santo is fairly good even in winter (with about 5 hours of sunlight per day in November, December and January), while it is good in the summer months, when the sky is often (though not always) clear.
In June, a cloud cover often forms (called capacete) over the island in the early morning, but thins out during the warmest hours.
In the period from June to September, Porto Santo can be affected by the Leste, a hot, 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 reaches 20 °C (68 °F) in June, when it's therefore still very cool, and 23 °C (73 °F) in August and September, when it becomes warm enough for swimming.
Here are the average sea temperatures.
Sea temperature - Porto Santo
Porto SantoJanFebMarAprMayJunJulAugSepOctNovDec
Sea (°C)181818181920212323222119
Sea (°F)646464646668707373727066

Having a very mild climate, Porto Santo can be visited all year round, but from mid-October to mid-April there may be rains and thunderstorms; the best time is therefore the summer (July to September) for the sun and the sea, while to visit cities and make excursions you can choose the spring, from mid-April to June.

In suitcase - what to bring

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