Two unique types of waves ripple through the Indian Ocean in this spectacular true-color Moderate Resolution Imaging Spectroradiometer (MODIS) image, taken by the Terra satellite on November 11, 2003. In the upper western portion of the image, atmospheric gravity waves reveal themselves in double, overlapping arcs of clouds. In the eastern part of the scene, a mixture of wave patterns caused by atmospheric gravity waves and internal ocean waves fans out from the coast. Sunglint (sunlight reflecting off of rough surface waters and back into MODIS’s “eye”) slants across this portion of the image, which makes the different types of waves easier to see.
Atmospheric gravity waves (also called atmospheric internal waves) occur when a uniform layer of air blows over a large obstacle, like a mountain or island. Before hitting the obstacle, the atmosphere must be stratified — each layer must have a uniform temperature and density that only changes with height. When the air hits the obstacle, the horizontal ribbons of uniform air are disturbed, which forms a wave pattern. This wave pattern in the air impresses itself onto sea waves when it touches the surface of the ocean. In addition to the surface mimicking the wave pattern, wave clouds can form as well.
Internal waves happen much in the same way that atmospheric gravity waves do; the main difference being that the waves occur between layers of water with different densities instead of layers of the atmosphere. When the bottom layer of dense water encounters an obstacle, a wave pattern forms at the point where the bottom layer of water and the next, lighter layer meet. These waves don't actually ripple the surface of the water, but they become visible because of the way light reflects off of them.