Weather changed with the seasons
The weather in ancient Greece, like the weather in most places, changed with the seasons. Greek weather also depended on whether you were in Northern Greece (near Thermopylae) or in Southern Greece (near Sparta).
Spring weather in Greece
Summer in ancient Greece
In Northern Greece sometimes there are thunderstorms in the summer. In Northern Greece it would generally be in the 80s or 90s Fahrenheit (about 30 degrees Celsius), during the day but in Southern Greece it could get up over 100 pretty often (over 40 degrees Celsius).
Fall weather in Greece
In the fall, around October, it would begin to rain again, and there would be storms. The autumn temperature would drop into the 70s and then into the 60s Fahrenheit (25-10 degrees Celsius). This would be the time to plant the wheat and barley.
Did it snow in the winter?
Most of the time in Southern Greece it would be in the 50s during the winter (about 10 degrees Celsius). You would need a cloak to be comfortable outside, but you wouldn’t need mittens.
In Northern Greece, it would be colder, and there would probably be snow several times in a winter, though you wouldn’t have snow on the ground continuously. You would need a warm cloak, and boots, and mittens, to keep warm outside (In ancient Greece they didn’t have sweaters, because knitting hadn’t been invented yet).
Climate change in ancient Greece
There were many small changes in the climate in Greece between the Stone Age and today. Some of those changes may have been natural, like the Little Ice Age, and others may have been caused by things people did, like farming, making charcoal, or cutting down trees to burn to make iron. Today with global warming, like everywhere else on Earth, Greece is getting warmer.
Learn by doing: compare the weather in Greece to your weather for a few days
More about the Greek environment
Greece the Land (Lands, Peoples & Cultures), by Sierra Adare (1998). Geography, climate, and more.
Ecology of the Ancient Greek World, by Robert Sallares (1991). Not easy going, but very interesting.