Gravity Wave Banding and "Streets" off Mozambique seen from space

August 16, 2002, shows various cloud streets in the Mozambique Channel between Madagascar (east) and southeast Africa. Cloud streets typically form along the path of low-level winds when they blow over open water, which is typically warmer than the wind itself.

Warm air from near the ocean surface rises (convects) and is then swept along by the wind. The air is thus subject to two competing influences: it’s rising up, but it is also being pushed along by the wind. These two influences cause the rising air to roll and spin, producing a row of horizontal vortices (imagine a tornado on its side) all lined up in the direction of the wind.

The vortices do not all spin the same way; in fact, adjacent vortices spin in opposite directions. To picture this, point the index finger of each of your hands toward the computer. Now trace a clockwise circle in the air with your right hand, and a counter-clockwise circle with your left hand at the same time. Where your fingers come closest together, both of them are moving upward. If your fingers were vortices, the air in each vortex would be rising right next to each other, which is where the clouds form. Where your fingers are farthest apart is where air would be spinning down toward the Earth, and no clouds form there.

This alternating pattern produces the lines of clouds (streets) like those seen in this image. In this case, some of the streets appear to be making a series of concentric circles, which reveals the wind direction.

Coastal fog commonly drapes the Peruvian coast. This image captures complex interactions between land, sea, and atmosphere along the southern Peruvian coast. When Shuttle astronauts took the image in February of 2002, the layers of coastal fog and stratus were being progressively scoured away by brisk south to southeast winds. Remnants of the cloud deck banked against the larger, obstructing headlands like Peninsula Paracas and Isla Sangayan, giving the prominent “white comma” effect. Southerlies also produced ripples of internal gravity waves in the clouds offshore where warm, dry air aloft interacts with a thinning layer of cool, moist air near the sea surface on the outer edge of the remaining cloud bank. South of Peninsula Baracas, the small headlands channeled the clouds into streaks—local horizontal vortices caused by the headlands provided enough lift to give points of origin of the clouds in some bays. Besides the shelter of the peninsula, the Bahia de Pisco appears to be cloud-free due to a dry, offshore flow down the valley of the Rio Ica.

In this natural-color image from the Multi-angle Imaging SpectroRadiometer (MISR), a fingerprint-like gravity wave feature occurs over a deck of marine stratocumulus clouds. Similar to the ripples that occur when a pebble is thrown into a still pond, such “gravity waves” sometimes appear when the relatively stable and stratified air masses associated with stratocumulus cloud layers are disturbed by a vertical trigger from the underlying terrain, or by a thunderstorm updraft or some other vertical wind shear. The stratocumulus cellular clouds that underlie the wave feature are associated with sinking air that is strongly cooled at the level of the cloud-tops—such clouds are common over mid-latitude oceans when the air is unperturbed by cyclonic or frontal activity. This image is centered over the Indian Ocean (at about 38.9° South, 80.6° East), and was acquired on October 29, 2003.

Since the fluid is a continuous medium, a traveling disturbance will result. In the earth's atmosphere, gravity waves are important for transferring momentum from the troposphere to the mesosphere. Gravity waves are generated in the troposphere by frontal systems or by airflow over mountains. At first waves propagate through the atmosphere without affecting its mean velocity. But as the waves reach more rarefied air at higher altitudes, their amplitude increases, and nonlinear effects cause the waves to break, transferring their momentum to the mean flow.

This process plays a key role in controlling the dynamics of the middle atmosphere.

The clouds in gravity waves can look like Altostratus undulatus clouds, and are sometimes confused with them, but the formation mechanism is different.

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 affected area of the image, smooth ocean water becomes a silvery mirror, while rougher surface waters appear dark. Sometimes the sunglint region of satellite images reveals interesting ocean or atmospheric features that the sensor does not typically record. This image shows a large, overlapping wave pattern in the sunglint region of an image of Indonesia (the islands at the top of the image) and Australia (the landmass in the bottom of the image). The wave pattern seen in the image is not from large ocean waves, however. The pattern is the “impression” of atmospheric gravity waves on the surface of the ocean. As the name implies, 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 towards the sensor. Clouds commonly form at the crests of the waves, and such clouds are visible throughout this scene.