On December 21, 2010 the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Terra satellite flew over the Southern Pacific Ocean, capturing this unusual true-color image of atmospheric gravity waves off the eastern coast of South Island, New Zealand. This type of wave is normally invisible to the MODIS sensor, except when the phenomenon occurs in an area of sunglint.
Sunglint occurs when the Sun reflects off the surface of the ocean at the same angle that a satellite sensor is viewing the same surface. In the sunglint area, smooth ocean water reflects light almost directly back to the sensor. Such an area looks like a silvery-gray mirrored surface, while roughened waters scatter light, giving a blue or black appearance.
In this image, it appears as if very large ocean waves are rolling towards the southeast from the shoreline. In fact, it is not the water that is rolling, but the waves are occurring in the air above the ocean. Where the bottom of the atmospheric wave touches the water below, an imprint becomes visible.
Atmospheric gravity waves form when buoyancy pushes air upwards, while gravity pulls it down in an oscillating pattern. At the low point of the wave (the trough), the air touches the surface of the ocean, roughening the otherwise smooth water. Where the troughs touch the water, less light is reflected directly back towards MODIS. When the area is in an area of sunglint, the background, formed by smooth water, is silvery-gray while dark lines appear in the roughened area. Together, the silver and blue pattern looks like waves.