Map from Google - Sicily

In Sicily, the largest island in Italy and the Mediterranean with its 25,711 square kilometres, the climate is Mediterranean along the coasts and in the islands, with mild and moderately rainy winters, and hot, sunny summers. In inland areas, the climate becomes slightly continental at hill altitudes, where winters become moderately cold and summers are still hot (and often torrid), while in mountain areas it becomes colder, although at the same altitude it's far less cold than in the Alps or the Apennines.
The highest mountain is the volcano Etna, located in the eastern part of the island, 3,329 metres high, and the top, which is located just 25 km away from Catania, in winter, and often also in spring and autumn, it is covered with snow. Almost the whole interior of Sicily is occupied by hills and mountains; in particular in the north we find some mountain ranges parallel to the coast: the Madonie (whose highest peak is Pizzo Carbonara, 1,979 metres high), the Nebrodi (whose highest peak is Mount Soro, 1,847 metres), and the Peloritani (which reach 1,374 metres). In the centre and south of the region there are mostly low hills and mountains, arriving at about 1,000 metres above sea level, with the exception of the Sicani Mountains, about 30 km south of Palermo, which arrive at 1,613 metres in the highest peak, Rocca Busambra.
Above 1,000 metres, rainfall is more abundant than at lower altitudes, so forests grow there.

Mount Etna as seen from Catania

On the contrary, precipitation is usually not abundant at low altitudes, ranging from 400 to 600 millimetres per year (eg rainfall amounts to 495 mm per year in Trapani, 525 mm in Syracuse, 545 mm in Catania, 555 mm in Enna, 610 in Palermo, and just 370 mm in Gela, on the southern coast). Messina is an exception, with 850 mm per year. The performance of the rains in Sicily is Mediterranean, with a maximum in autumn and winter, when even thunderstorms are fairly frequent, a drop in spring, when there is already little rain, and a minimum in summer, when it rains very rarely. The landscape is often semi-arid, also because of the long summer drought, especially at low altitudes and on the southern slopes.
The island is subject to the sirocco, the hot wind from Africa, which can lead to temperatures around or above 20 degrees in winter, and 40 °C in summer. During the days with the most intense sirocco, the temperature reached values of up to 43 °C in Messina, and 44/45 °C in Trapani, Palermo and Catania.
Another frequent wind is the fresh mistral, blowing from the northwest and affecting mainly the western part of the island.
Winter cold spells, which come from the Balkan Peninsula, usually do not last long in Sicily, two or three days, and typically are not intense along the coasts, where cold records are around 0 °C, and snowfalls are very rare (occasionally it can snow in Palermo and Messina, on the north coast, while on the south coast in practice it never snows). In the hills of the interior, however, snow is a little more frequent, while in the mountainous areas above a thousand metres, it can sometimes be abundant.
An exceptional snowfall occurred on December 31, 2014, when snow occured in areas where it is almost never seen, as in Cozzo Spadaro and Pachino, on the southeastern tip of the island.
The amount of sunshine in Sicily is great in summer, when clear skies prevail; in winter, sunny periods alternate with periods of bad weather, with rain, wind and thunderstorms.

Now let's see the data of some cities, starting from the coasts.


Palermo, the cathedral

Palermo, the region's capital, overlooking the southern Tyrrhenian Sea, like the rest of the Sicilian coast has very mild winters and hot summers. The city is prone to the sirocco, which is able to increase the temperature by several degrees, but also to the wind that comes from Sardinia. During the summer, when sunny days prevail, the sea breeze tempers the heat.
Here are the average temperatures.
Average temperatures - Palermo
PalermoJanFebMarAprMayJunJulAugSepOctNovDec
PalermoJFMAMJJASOND
Min (°C)9910111519222320171310
Max (°C)151516192327303128241916

Rainfall amounts to 615 millimetres per year, with a maximum in autumn and winter, and a minimum in summer, when it almost never rains.
Here is the average precipitation.
Average precipitation - Palermo
PalermoJanFebMarAprMayJunJulAugSepOctNovDecYear
PalermoJFMAMJJASONDY
Prec. (mm)90706045251051040808595615
Days12997421158101280

The north-western coastal towns that are located in more open areas, from Mondello to Isola delle Femmine, and from Terrasini to San Vito lo Capo, are in summer a bit cooler and windier, so much so that the maximum temperatures in July and August are about 28/29 degrees on average.

San Vito lo Capo

In Trapani, and in other locations on the west coast (see Marsala, Mazara del Vallo), the climate is similar to that of Palermo, as well as in the rest of the north noast (see Cefalu, Milazzo).
On the north-eastern tip of Sicily we find Messina, which has a particularly warm microclimate, being closed between the Strait and the Peloritani mountains. You can notice in particular the high minimum temperatures.
Here are the average temperatures.
Average temperatures - Messina
MessinaJanFebMarAprMayJunJulAugSepOctNovDec
MessinaJFMAMJJASOND
Min (°C)101011131620232422181412
Max (°C)141516182327303128231916

For its location, Messina is also particularly rainy compared to other coastal and low-lying areas of Sicily: precipitation amounts to 850 mm per year, with 100 mm per month or more from October to February. However, even here in summer it rains very rarely.
Here is the average precipitation.
Average precipitation - Messina
MessinaJanFebMarAprMayJunJulAugSepOctNovDecYear
MessinaJFMAMJJASONDY
Prec. (mm)10510085703515202565115120105850
Days111099422369111187

Taormina is on the east coast, halfway between Messina and Catania, 200 metres above sea level, at the foot of a hill 400 metres high, with panoramic views of the sea and Mount Etna in the background. At 3 km south of Taormina we find Giardini-Naxos.

Taormina, theater, sea and Mount Etna in the background

On the east coast of Sicily we find Catania, which is located in the only plain of some importance in the region, the plain of Catania. Here the temperature range becomes slightly higher, so that nights are a bit colder in winter, while in summer it's one of the areas of Italy where the maximum temperatures are higher. While in Catania, which lies on the coast, the average maximum temperature in July and August is 32 degrees, in Sigonella, 15 km from the coast, it arrives at 33 °C, and in Lentini at 34 °C. Even the highest records are remarkable: at the Fontanarossa airport the temperature touched 45 degrees in June 1982 and 46 degrees in July 1962, while in Catenanuova, about forty kilometres away from the coast and in the province of Enna, it touched even 48 degrees, though in a not officially recognized weather station.
Here are the average temperatures in Catania.
Average temperatures - Catania
CataniaJanFebMarAprMayJunJulAugSepOctNovDec
CataniaJFMAMJJASOND
Min (°C)5578121619191714107
Max (°C)161618202428323229252017

In Catania, 550 mm of rain per year fall, with a maximum in autumn, and as usual, a pronounced minimum in summer.
That's the average rainfall.
Average precipitation - Catania
CataniaJanFebMarAprMayJunJulAugSepOctNovDecYear
CataniaJFMAMJJASONDY
Prec. (mm)75554535205510451056585550
Days87653112375856
 
Further south of Catania, and still on the east coast, we find Syracuse, where the climate is similar, although the night temperatures are slightly higher.
Here are the average temperatures.
Average temperatures - Syracuse
SyracuseJanFebMarAprMayJunJulAugSepOctNovDec
SyracuseJFMAMJJASOND
Min (°C)78911141821211916129
Max (°C)151517202428313128242016

Even precipitation in Syracuse is similar in both amount and pattern to that of Catania.
Here is the average precipitation.
Average precipitation - Syracuse
SyracuseJanFebMarAprMayJunJulAugSepOctNovDecYear
SyracuseJFMAMJJASONDY
Prec. (mm)755045301553745809580525
Days97643111478960

On the south coast of Sicily, from Sciacca to Porto Empedocle (near Agrigento), and from Licata to Gela, the climate is similar to that of Syracuse, although the rains are even less abundant, as they hover around 400/450 mm per year, and in some cases they drop even below 400 mm.

The sea in Sicily is warm enough for swimming from July to September. In June, the sea is still a bit cool, while in October, after the summer, its temperature is still decent (23 °C): in fine days, perhaps with the sirocco, Sicilians go to the beach. Here for example, the temperature of the sea at Palermo (the same as in the rest of the region).
Sea temperature - Palermo
PalermoJanFebMarAprMayJunJulAugSepOctNovDec
PalermoJFMAMJJASOND
Sea (°C)151515151821242625232017

Inland areas

With the exception of the already mentioned plain of Catania, Sicily's inland areas are occupied by hills and mountains. Here the winter gradually becomes colder with increasing altitude, and above 800 metres (and sometimes even 500 metres) during cold spells it can snow; sometimes fog can form. In summer, however, something special happens: in the first few hundred metres of altitude, the temperature does not decrease, indeed it may be even higher than on the coasts, since the mild influence of the sea here is absent or reduced, thus there are often torrid days. Precipitation is generally quite low, around or below 500 mm per year, while it increases again at higher altitudes, above 1,000 metres. As always, however, summer is the driest season.
Here for example, the average temperatures of Ragusa, the capital of the southernmost province of Italy, 500 metres above sea level.
Average temperatures - Ragusa
RagusaJanFebMarAprMayJunJulAugSepOctNovDec
RagusaJFMAMJJASOND
Min (°C)446811161819161295
Max (°C)121315172329323228221814

In Caltanissetta, at 570 metres above sea level, temperatures are very similar to those of Ragusa.

Enna

To find lower temperatures even in summer we have to climb further in altitude. In Enna, the highest provincial capital of Italy, located in the centre of Sicily and about 900 metres of altitude, in autumn and winter fog often forms, while in summer the heat is generally bearable (but when the south wind blows, the temperature can reach 35/36 °C). In winter, during cold spells, sometimes it can snow and freeze.
Here are the average temperatures of Prizzi, located further west, at 1,000 metres above sea level (the temperatures are similar to those of Enna).
Here are the average temperatures.
Average temperatures - Prizzi
PrizziJanFebMarAprMayJunJulAugSepOctNovDec
PrizziJFMAMJJASOND
Min (°C)223510141717141064
Max (°C)781013192427272317128

In Prizzi, 600 mm of rain per year falls, as always with a maximum in autumn and winter, and a minimum in summer.
Here is the average precipitation.
Average precipitation - Prizzi
PrizziJanFebMarAprMayJunJulAugSepOctNovDecYear
PrizziJFMAMJJASONDY
Prec. (mm)65605055301081550808585600
Days10988421258101178

When to go

The best time to visit Sicily for a beach holiday is the 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), the first periods of bad weather begin to occur. 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 cool 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.

For the islands of Sicily, see:
Aegadian Islands
Aeolian Islands
Lampedusa
Pantelleria
Ustica