As the month has progressed, the weather in Greece has definitely changed. I'm glad I brought some warmer clothes and a rain jacket.
Athens, 11-16 Oct. Sunny, hot and humid during the day. Cool in the evening. Some mosquitoes. Kept the apartment closed up with the AC on while there.
Fira (Santorini), 16-20 Oct. Sunny, hot and humid until late afternoon, long before sunset. Very windy all days, all times of day. Cold in the wind after about 16:00.
Heraklion (Crete), 20-22 Oct. Very hot and sunny until late afternoon. Then cold and windy.
Chania (Crete), 22-25 Oct. Heavy rain with sunbreaks. Very windy all day and cold at night.
Hydra, 26 (now) Oct. Sunny, but very windy and cold.
I'm posting this in real time as on the ground info instead of averages. Tomorrow, next week, next month it could all change, but at least for these 2 weeks, there is a pattern.
The wind has certainly affected our ferry rides (Flying Cat from Fira to Heraklion, overnight Anek Kriti II from Chania to Piraeus, Flying Dolphin from Piraeus to Hydra). Fortunately, I brought Dramamine for my husband (Navy reservist for 12 years) who is sure I scheduled the boats just to torment him.
The wind also affected our flight from Athens to Santorini. The pilot had to bank the plane severely because of it.
I'm very glad I brought layers good for all the weather we've experienced. With the hoods on our rain jackets, we've been able to avoid the umbrella vendors that sprout up like mushrooms at the first drop.