The Aegean Sea wore a mottled, silvery sheen when seen from space in mid-July 2020. The Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASA’s Terra satellite acquired a true-color image of the scene on July 14.
The dynamic streaks of silver were caused by a combination of wind and an optical phenomenon called “sunglint”. Sunglint occurs where the surface reflects sunlight directly back to the satellite’s imaging instrument. This creates a mirror-like silver shine in the image – at least where sea surfaces are smooth. In areas where the surface water is turbulent or roughened, sunlight scatters in many more directions, so the water appears darker.
Each year, from May until about September, strong, dry winds blow over the Aegean Basin. Known as Etesian or meltemi winds, these breezes help cool the islands in summer’s heat. As the winds blow over the blue Aegean waters, they ripple and roughen the surface. When the wind strikes one of the myriad islands found in the basin, the water on the windward side piles up against the shore, becoming rougher. The leeward side, however, sits in a “wind shadow” —an area protected from the strong winds created by the height of the island. The waters in the wind shadow remain calm and smooth, making them a perfect surface for sunglint.
The direction of the wind can be easily estimated from the long silver trails of sunglint. It is blowing towards the southwest in the northern part of the image and arcs generally to the southeast in the midsection, and almost due south past the large island of Crete.