Greece's climate varies according to region. The southern and central portions of the country experience the traditional mediterranean climate of hot, dry summers and mild, wet winters. The continental climate of northern Greece is marked by colder winters.
There is also considerable regional variation in annual rainfall, with areas to the west of the Pindus Mountains receiving far more rain than those to the east.
The climate in Santorini  is deceptive. It's similar to the Bay Area in California, mildly hot during the day, but cooler at night.  The climate is mild and dry. During the summer, temperatures varry between 25 to 35 degrees.
March, April and May, often provide warm sunny days with the occasional unpredictable change of temperature.
June and September, are mostly sunny with moderate to hot temperatures.
July and August, are the hottest months and the days are dry with frequent cool breezes in the evenings. During those months it is also common to encounter sudden winds called "Meltemi" in the Aegean Sea. They start suddenly and blow continuously for some days. One can also perceive them, as the natural air conditioning for the hot months.
October to February, the temperature is quite low with reasonable amount of rain.
Below one can see the various measurements.
Weather / Climate             Temperatures
January            08-15°C (46-60°F) February          08-15°C (46-60°F) March              08-15°C (46-60°F)
April                 16-22°C (61-72°F) 
May                 16-22°C (61-72°F)
June                 23-30°C (73-86°F) 
July                  23-30°C (73-86°F) 
August            23-30°C (73-86°F)
September     16-22°C (61-72°F)
October          16-22°C (61-72°F)
November       16-22°C (61-72°F) December       08-15°C (46-60°F)