Sicily have a Mediterranean climate with hot, dry summers and warm, rainy winters. The island is volcanic, the tallest and most active volcanoe in the world is Mount Etna. The mountainous are the reason for great regional variation in climate depending on altitude and the protection they provide from northwest prevailing winds. The southeast regions are generally drier and warmer than the northwest and the southeast coast is usually calmer. Higher areas receive cooler temperatures and higher rainfall arround the year.
Summer. From June untill September it is hot and dry with a lot of sunshine. The average high temperature is already at 25°C / 77°F in June, 27°C in July and 28°C / 82°F in August before going down to 26°C / 79°F in September. By night it is also very warm, arround 20°C - 68°F. The water temperature gets up to 28°C / 82°F in August. Along the coastline the the breeze makes you stay very comfortable and you can enjoy the medium of 10 sunshine hours. Rain is very rare in this period.
Sometimes the scirocco, a very hot wind from Sahara, can make you forget what I've been written here before. Than, without a cool breeze from the sea, it will be just hot. Hot, dusty, windy and with an angry yellow sunsets because the air is full of sand from the desert Sahara. But after two, three days everything stops with a short rainfall, covering everything with fine yellow powder-like sand and the weather turns as it was before.
Autumn. In October and November the summer-like weather continues. October with average highs between 20-25°C / 68-77°F. It only drops below 20°C / 68°F in mid November though some cool days may occur. Rainfall does increase but cloud cover and these brief showers still leave seven hours of sun per day in October and five in November. The sea remains warm enough for swimming untill the very end of the season. From December on it starts snowing on Mount Etna and the ski season beginns.
Winter. From December to February, the winter is mild but unpredictable. The sea is never getting colder than 12°C / 54°F, that's why it cannot freeze along the coastline. Snow we have only above 200 Meters. There are some ski resorts on the Mount Etna and in the Madonie mountains. If you don't like skiing, now it's a good time for walking in the lower parts of Sicily and jumping into the warm sulphur water springs. Average temperatures are between 10 and 15°C / 50 and 59°F. If you're lucky the sun shines and if not, it might be raining..
Spring. From March to May it is warm and sunny. The average temperatures from 16°C / 61°F in March, 18°C / 64°F in April and up to 22°C / 72°F in May. Best time for excursions and round trips. Snow on the mountains melts in March, making end of the ski season.