On October 27, 2010, the Moderate Resolution Imaging Spectroradiometer (MODIS)aboard the Terra satellite passed over northwestern Australia, capturing this unusual image of the atmosphere’s interaction with the calm waters of the Indian Ocean.
Although the large-scale arcing pattern appears to be created by waves rolling though water, in fact the silvery waters are quite still. The air above the water, however, is rising and falling in an undulating pattern, touching the water at regular intervals. Because this occurs in an area of sunglint, the otherwise invisible interaction of atmosphere on ocean becomes spectacularly visible.
When the Sun reflects off the surface of the ocean at the same angle that a satellite sensor is viewing the surface, a phenomenon called sunglint occurs. In the sunglint area of a MODIS image, smooth water becomes a silvery mirror, while rougher surface water appears dark.
Atmospheric gravity waves form when buoyancy pushes air up, and gravity pulls it back down. On its descent into the low-point of the wave (the trough), the air touches the surface of the ocean, roughening the water. The long, vertical dark lines show where the troughs of gravity waves have roughened the surface. The brighter regions show the crests of the atmospheric waves. Beneath the crests, the water is calm and reflects light directly back toward MODIS. Clouds commonly form at the crests of the waves, and such clouds are visible over the northwestern most waves while the more southerly waves remain cloud-free.