NASA MODIS Image of the Day: November 24, 2012 – Cloud vortices off Saint Helena Island, south Atlantic Ocean
It was the strong southeasterly trade winds which brought sailing vessels round the Cape of Good Hope and across the South Atlantic Ocean, and led to the discovery of the remote Island of St. Helena in the early years of the sixteenth century. In the twenty-first century, a different sort of vessel rounds the horizon far overhead, but the same southeast trade winds can still bring attention to the small island lying in the remote ocean, especially in cloud-filled skies. NASA’s Terra satellite passed over the South Atlantic Ocean on November 15, 2012, allowing the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument flying aboard to capture this true-color image of St. Helena Island and the band of wind-blow cloud vortices trailing towards the island’s leeward side. St. Helena Island is a tiny island lying approximately 1,860 kilometers (1,156 miles) west of Africa. Volcanic in origin, it has rugged topography with steep, sharp peaks and deep ravines. Wind, which can blow unimpeded for hundreds of miles across the ocean, strikes the face of the mountains, and is forced around the unyielding terrain. As it blows around the island, the air spins on the leeward side, much like a flowing river forms eddies on the downriver side of a piling. The spinning wind forms intricate – and mathematically predictable – patterns. When clouds are in the sky, these beautiful patterns become visible from above.
