Internal waves off Libya
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Internal waves off Libya

September 15, 2004
In the waters of the Mediterranean Sea, off the coast of Libya, faint grey lines curving through the silver swath of sunglint mark the passage of internal waves. As the name implies, internal waves occur not at the surface of the water, but inside it. Often these waves are visible only from great heights, such as those at which satellites travel. The Terra MODIS instrument, which acquired this true-color image on July 12, 2004, travels at an altitude of 705 km above the Earth's surface. The Mediterranean Sea, like all major bodies of water, is composed of layers of water with differing densities. The topmost layer is the least dense, with each successively deeper layer being denser. Internal waves move along underwater at the boundary between layers of different densities. Internal waves are usually caused by the lower layer being forced against a shallow obstacle, like a ridge, by tidal action. The ridge causes a disturbance, which creates a wave in the water layer, similar to

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