This astronaut photograph illustrates the formation of wave clouds in the wake, or downwind side, of Île aux Cochons (“Pig Island”) in the southern Indian Ocean. The island is located approximately 3,000 kilometers (1,900 miles) southeast of South Africa and 2,300 kilometers (1,400 miles) northwest of Antarctica. In this view from the International Space Station, only a part of the eastern coastline is visible.
The island is volcanic in origin and has a summit elevation of 775 meters (2,543 feet) above sea level. The Île aux Cochons stratovolcano is thought to have erupted within the past 12,000 years; however, no historical activity has been recorded.
The summit elevation is high enough for the land surface to interact with cloud layers and with winds flowing past the island. In this image, two cloud layers are visible. The lower, more uniform layer consists of roughly parallel “cloud streets” that suggest the winds blowing out of the west. When air masses run into the summit of Île aux Cochons, moisture-laden air rises and cools, causing water vapor to condense into clouds.
Once the air masses pass over the summit, they descend and may encounter alternating moist and dry air layers, enabling the formation of the discontinuous, chevron-shaped wave clouds. While their appearance suggests that the clouds are forming in the wake of the island and moving eastwards, it is in fact the air mass that is moving, with clouds forming in regions of moist air and dissipating in dry regions.
Île aux Cochons is the westernmost of the islands in the sub-antarctic Crozet Archipelago, part of the French Southern and Antarctic Lands. Except for occasional research visits, the island is uninhabited. The island is an important breeding site for seabirds, including the world’s largest King Penguin colony.