Map from Google - Santorini

The climate of Santorini, the largest and southernmost of the Cyclades, is typically Mediterranean, with mild winters and warm and sunny summers. The Cyclades (which include islands like Delos, Mykonos, Paros and Ios) are located in the Aegean Sea.
Here are the average temperatures.
Average temperatures - Santorini
SantoriniJanFebMarAprMayJunJulAugSepOctNovDec
SantoriniJFMAMJJASOND
Min (°C)9910121519222219161311
Max (°C)141416192227282726221916
Min (°F)484850545966727266615552
Max (°F)575761667281828179726661

The rainfall pattern is typically Mediterranean, so that the rains are concentrated in autumn and winter, in particular from October to March. In practice, it rains very rarely from April to September, and almost never in summer, from June to August. The length of the dry season makes the landscape arid. The interior is largely flat, but there is also a hill, Mount Profitis Ilias, 567 meters high.
Here is the average precipitation.
Average precipitation - Santorini
SantoriniJanFebMarAprMayJunJulAugSepOctNovDecYear
SantoriniJFMAMJJASONDY
Prec. (mm)11580653015511156070100557
Prec. (in)4.53.12.61.20.60.2000.62.42.83.921.9
Days97632100135946

In the warm season, starting from May, the Cyclades (and therefore also Santorini) are particularly affected by the Meltemi, a wind that blows from the north, persistently and in periods that last a few days. This wind is able to make the air feel cooler than it is, but it can be also annoying and make the sea rough.

In winter, from December to mid-March, the temperatures are generally mild, but waves of bad weather, with wind and rain, can not be excluded. Sometimes there can be cold days, with the north wind blowing and highs remaining below 10 °C (50 °F).

In spring, from mid-March to May, the weather is mild, and it gradually becomes more stable, with rain gradually less frequent.

Summer, from June to mid-September, is warm and sunny. The wind almost always makes the heat bearable, although sometimes there can be some short heat waves of African origin.

Autumn, from mid-September to November, is initially warm and sunny, but then the first rains occur, and with each passing week they become more frequent. In autumn there may be many sunny days, but the rains in this season can sometimes be abundant.

The amount of sunshine in Santorini is great in summer, when clear skies are the norm. In winter, periods of bad weather, with cloudy skies, alternate with sunny periods.

Santorini

The sea is warm enough for swimming from July to September, reaching a maximum of 24 °C (75 °F) in August: therefore it is not very warm, but it is acceptable.
Here are the average sea temperatures.
Sea temperature - Santorini
SantoriniJanFebMarAprMayJunJulAugSepOctNovDec
SantoriniJFMAMJJASOND
Sea (°C)161515161821232423211917
Sea (°F)615959616470737573706663

When to go

The best time for a beach holiday in Santorini is the summer, from June to mid-September. Those who are sensitive to cold can bring a sweatshirt, even in midsummer, for windy and cool evenings.
In spring and autumn, especially in April and May and from mid-September to mid-October, you will find pleasant temperatures, and the weather will be often sunny, although there can be also some short periods of bad weather.

Back to the Cyclades Islands.