Prince Edward Islands wave clouds
The Prince Edward Islands are two small islands in the sub-antarctic Indian Ocean that are politically part of South Africa. They are located about 1770 km south-east of Port Elizabeth, in mainland South Africa. The two islands are named Marion Island and Prince Edward Island. Their only human inhabitants are the staff of a meteorological and biological research station run by the South African National Antarctic Programme on Marion Island.
Both islands are of volcanic origin. Marion Island is one of the peaks of a large underwater shield volcano that rises some 5000 m (16,500 ft) from the sea floor to the top of Mascarin Peak. The volcano was thought to be extinct, but erupted in 1980 and is now classified as active.
The islands are among the cloudiest places in the world, and it rains on average about 320 days a year (about 28 days a month). Although the islands are covered by clouds in the image, their location is clearly visible at the apex of the clouds with the form of waves from a ship’s wake. These wave cloud patterns form as stable air flows over a mountain range, and can either form above the range or on the side protected from the wind. As an air mass travels through the wave, it undergoes repeated uplift and descent. If there is enough moisture in the atmosphere, clouds form at the crests of these waves. In the descending part of the wave this cloud evaporate due to adiabatic heating, leading to the characteristic repeating cloud/clear bands. The cloud base on the leeward side (the side sheltered from the wind) is higher than on the windward side because precipitation on the windward side removes water from the air.