This wide field-of-view image from the International Space Station shows an east-west swath of the southwestern Indian Ocean around at 50° South latitude. Two remote islands, part of the French Southern and Antarctic Lands, appear in the center of the image. Possession Island and East Island are each just 18 kilometers (11 miles) long. A smaller island, Île aux Cochons (Pigs Island), lies 100 kilometers (60 miles) to the west. Each island has set up V-shaped trains of waves, like bow waves, as the air flows from the west (left to right). The bow-wave patterns are overlaid on the low regional stratus (blanket) clouds that are so common in the southern Indian Ocean.
This photograph was taken by an astronaut who was more than 400 kilometers (250 miles) above the sea surface, and it reveals relationships that could not be readily understood by someone standing on one of the islands. For example, larger and higher islands produce larger waves, so the largest are being generated by Possession Island (934 meters above sea level at the highest point) and East Island. Much smaller waves develop downwind of the tiny Île de Pingouins (340 meters above sea level).
Other patterns also can be detected. Waves in an upper cloud layer cast shadows onto a lower layer (image top right). In the bottom half of the image, the waves make thicker and thinner zones in the clouds of the lower layer. And wave trains from Possession Island and Île aux Cochons are interacting in a cross-hatch pattern.
Wave clouds generated by Île aux Cochons were captured by astronauts in 2012 in a more detailed view, taken with a 180 mm focal-length lens, compared with this wide 28 mm lens view.
All islands in the image have been designated IBA’s (Important Bird Areas) because they are breeding sites for numerous species of seabird. Interestingly, the pigs once introduced to Île aux Cochons have since been eradicated to help protect bird habitats. Introduced goats have been eradicated on Possession Island, though rats remain and prey on smaller birds who now must nest at higher elevations for protection.