The Greek islands enjoy a warm and sunny Mediterranean climate. Everywhere the summers are hot and dry with brilliant light and clear skies. Temperatures are in the 80s, tempered only by the northern meltemi winds which can blow fiercely for days at a time in August. Many people go in spring when the sun is more temperate and the wild flowers are in bloom. Winters are generally mild but islands can have snow and, when it does rain, it can tip it down in buckets.
The summer season in the Greek islands runs from May to October. The weather can be unpredictable out of season but,ironically it is best to book two weeks not one at this time of year. One week can be a washout and the next glorious but you rarely get two bad weeks in a row. If you are planning an early or late holiday think about the southerly islands such as Crete and Rhodes where the weather will generally be more settled.
Once the weather does settle - from late May to mid-September - it is almost guaranteed to be hot, bright and clear. Too hot sometimes, particularly on southern coasts of southern islands when tourist police can clear beaches at midday to prevent daft sunbathers getting grilled alive. Average sea level temperatures are 27° C in July. From late June to early August islands suffer from the brisk meltemi wind which, though it freshens the air, can make sunbathing on northern shores uncomfortable.
In winter low-pressure systems move in from the Atlantic, bringing rain while cold winds blow in from the Balkans. The same low-pressure systems draw warmer winds from the south so there can be a marked difference in winter temperatures between north and south islands. The Ionian Islands and the western mountains get most of the rain, often three times that in the sheltered east. The Ionian and Aegean islands are notably in October and November.
Timing your Greek island holiday can be tricky and much depends on what you plan to do when you get there. July is not a good time if you plan to do any hiking, early May can be a disaster for sunbathers and clubbers get only fag end of season entertainment in October.
The meltemi is a fierce wind that scours the islands throughout August. It results from high pressure over the Balkans and relatively low pressure over Turkey bringing strong, north east winds. It occurs mainly during the summer with July and August the worst affected months, but springs up occasionally in May and October. It usually starts in the early afternoon and can die out at sunset although, not uncommonly, it will last through the night and repeat for three to six, sometimes even ten, days. Northern islands are less affected than those in the south which can suffer from summer gales, particularly bad on north facing beaches. Ferry sailings and boat trips can be cancelled, sunbathing can be unpleasant and night sleep disturbed.