The Meltemi is the persistent northerly to northeasterly flow that affects the Aegean mainly from late spring to early autumn. In meteorology it is also known as the Etesian wind.
It is not a single gust or a local sea breeze. It is a large-scale flow that can last for several days and undergo strong local acceleration.
The pressure pattern
Summer low pressure over the hot eastern Mediterranean and Asia Minor often combines with higher pressure over the Balkans. The pressure gradient accelerates northerly flow across the Aegean.
When the eastern low and the anticyclone farther north both strengthen, the Meltemi can become strong or very strong.
Why local conditions differ
Channels between islands, capes and mountains accelerate or redirect the wind. A sheltered beach can be relatively calm while a windward coast only a few kilometres away has powerful gusts and waves.
Wind measured on land does not always describe the sea. Sailing and swimming decisions require a marine forecast, wave information and local observation.
The daily cycle
Wind often strengthens from late morning into the afternoon as daytime mixing brings faster flow down to the surface. An organised episode, however, can remain strong overnight.
The Meltemi often reduces heat along eastern coasts but increases fire-spread potential when vegetation is dry.
These explanations and calculations are educational. For safety, occupational, health, marine or aviation decisions, follow the appropriate official services.