Map from Google - Sardinia

In Sardinia, island of southern Italy, the climate is Mediterranean, with mild winters and warm and sunny summers, a bit sultry but tempered by sea breezes. In the plains and on the coast, the average temperature of January and February is about 10 degrees, while that of July and August is about 25 °C. The rains are not abundant, given that they typically fluctuate from 400 to 550 millimetres per year on the coast (but locally they can go even below, such as in the southeastern tip of the island), and follow a Mediterranean pattern, that is, they are more common in autumn and winter, and gradually decrease during spring, hitting a low in summer, when it almost never rains. In inland areas, rainfall exceeds locally 700 millimetres per year in hilly areas, and 1,000 mm in the mountains.
Here is the average precipitation of Cagliari, the main town, situated on the coast, in the southern part of the island.
Average precipitation Cagliari
Cagliari J F M A M J J A S O N D Year
Prec. (mm) 50 55 40 40 25 10 5 10 35 55 60 50 430
Days 7 7 7 7 4 2 1 1 4 7 7 7 61

Sardinia is a windy island, especially from October to April, because of the Maestrale, the northwest wind, which blows from France (where is called Mistral), and affects particularly the western part and the Strait of Bonifacio, but also because of the "Ponente" ("west wind"), which blows from the west. In summer, sea breezes blow, but they are welcome because they make the heat more bearable; on other occasions, the "Scirocco" can blow from the south-east, bringing heat waves of African origin, which are felt mainly in inland plains and valleys, with peaks of 40 °C and more.
In the interior the climate becomes gradually colder with increasing altitude, as there are hills and mountains, and in some areas as mentioned the rains are also more abundant, so that forests are found. The highest peak, 1,834 metres high, is located in the massif of Gennargentu, where in winter snowfalls are quite frequent.

Gennargentu

The island is quite sheltered from cold spells; along the coasts and on the plains, snowfall is quite rare, but can occur during the coldest winters (as in January 1985 and February 2012), more easily in the north of the island. An exception is the Costa Smeralda ("emerald coast", see Olbia), in the northeast, where snowfalls, though usually not abundant, are a bit more frequent. Night frosts in the plains are rare, and when they occur they are light: generally the temperature does not drop below -2/-3 °C. In inland areas instead, already at hill altitudes (see Nuoro, Macomer, Tempio Pausania), snow is relatively more frequent and abundant.
Here are the average temperatures of Alghero, located on the coast, in the north-west of the island.
Alghero average temperatures
Alghero J F M A M J J A S O N D
Min (°C) 6 6 7 8 12 15 17 18 16 13 9 7
Max (°C) 14 14 16 18 22 26 29 30 27 22 18 15

In the small islands and the most exposed coasts, the climate is particularly mild. Here, for example, the average temperature of Guardiavecchia, in the island of La Maddalena, where 470 mm of rain per year fall, with a maximum in December (75 mm), and a minimum in July (12 mm).
Guardiavecchia average temperatures
Guardiavecchia J F M A M J J A S O N D
Min (°C) 8 8 9 10 14 17 20 20 18 15 11 9
Max (°C) 13 13 14 16 19 23 27 27 24 20 16 13

In the inland plains the climate becomes slightly more continental. Here, for example, the average temperature of Decimomannu, about twenty kilometres north-west of Cagliari.
Decimomannu average temperatures
Decimomannu J F M A M J J A S O N D
Min (°C) 4 5 6 8 11 15 18 19 16 13 9 6
Max (°C) 14 15 17 19 24 28 32 32 28 23 18 15

As we said, at higher altitudes the weather gets colder. Here are the average temperatures of Fonni, located north of Gennargentu, at 1,000 metres of altitude.
Fonni average temperatures
Fonni J F M A M J J A S O N D
Min (°C) 2 2 3 5 9 13 16 17 13 10 6 3
Max (°C) 7 7 10 12 17 22 26 26 22 16 11 9

The sunshine amount in Sardinia is very good in the summer, especially along the coast, when clear skies prevail, while it's not so high from November to February; however some sunny periods can occur even in winter.

Caprera

The temperature of the sea is fairly uniform in the island, and just lower in the western part; the sea is warm enough for swimming from July to September; in June and October instead it's a bit cool. Here, for example, the temperature of the sea at Porto Cervo, on the Costa Smeralda.
Sea temperature Porto Cervo
Porto Cervo J F M A M J J A S O N D
Sea (°C) 15 14 14 15 17 20 23 25 24 21 18 16

When to go

For beach life, the best time to visit Sardinia is summer, from June to August. For swimming, the best time is from July to September, although in September, which in any case is still a good month (especially in the first half), you begin to see the first periods of bad weather. In June, as mentioned the sea is still a bit cool, while in May it's even cold, and sometimes even the air can be a little cold for sunbathing.
April and May are suitable for hiking and visiting cities, while in summer it can sometimes be too hot to get around with ease. In October, the temperatures are mild or pleasantly warm, but the days are shorter and the weather can be rainy and windy.