CRAZY CLOUDS: The Wave!

Clouds come in many shapes and forms. However, some of the most interesting come in the form of a wave!

Waves can occur in all fluids including in air. Actually, they are almost always occuring at some level of the atmosphere. However, they have to occur in the presence of clouds for us to see them.

That is exactly what occurred in Birmingham Alabama on December 16, 2011 when this rare "tsunami cloud" appeared over the city.

These clouds form in a process known as "Kelvin–Helmholtz instability" and they can closely resemble the types of waves that occur at the ocean shoreline.

According to Wikipidia: The Kelvin–Helmholtz instability (after Lord Kelvin and Hermann von Helmholtz) can occur when there is velocity shear in a single continuous fluid, or where there is a velocity difference across the interface between two fluids. An example is wind blowing over water: The instability manifests in waves on the water surface. More generally, clouds, the ocean, Saturn's bands, Jupiter's Red Spot, and the sun's corona show this instability.

The theory predicts the onset of instability and transition to turbulent flow in fluids of different densities moving at various speeds. Helmholtz studied the dynamics of two fluids of different densities when a small disturbance, such as a wave, was introduced at the boundary connecting the fluids.

Theoretically the above image describes the process with time expressed as "t".

Here's another dramatic example of a Kelvin–Helmholtz (K-H) instability cloud. This one occurred in Panama City Beach, Florida on February 5, 2012.

While the K-H cloud relies on a degree of instability to occur, other types of "wave clouds" form when the air is stable.

These are called "gravity wave clouds".

According to Wikipidia: In fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two media (e.g., the atmosphere and the ocean) which has the restoring force of gravity or buoyancy.

When a fluid element is displaced on an interface or internally to a region with a different density, gravity tries to restore the parcel toward equilibrium resulting in an oscillation about the equilibrium state or wave orbit. Gravity waves on an air–sea interface are called surface gravity waves or surface waves while internal gravity waves are called internal waves. Wind-generated waves on the water surface are examples of gravity waves, and tsunamis and ocean tides are others.

In the Earth's atmosphere, gravity waves are a mechanism for the transfer of momentum from the troposphere to the stratosphere. Gravity waves are generated in the troposphere by frontal systems or by airflow over mountains. At first, waves propagate through the atmosphere without appreciable change in 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.

Here's what a gravity wave looks like from space...

The "morning glory" is a very rare example of a type of wave cloud.

According to Wikipidia: The Morning Glory cloud is a rare meteorological phenomenon occasionally observed in different locations around the world. The southern part of Northern Australia's Gulf of Carpentaria is the only known location where it can be predicted and observed on a more or less regular basis. The settlement of Burketown attracts glider pilots intent on riding this phenomenon.

A Morning Glory cloud is a roll cloud that can be up to 1,000 kilometres (620 mi) long, 1 to 2 kilometres (0.62 to 1.2 mi) high, often only 100 to 200 metres (330 to 660 ft) above the ground and can move at speeds up to 60 kilometres (37 mi) per hour. Sometimes there is only one cloud, sometimes there are up to eight consecutive roll clouds.

The Morning Glory is often accompanied by sudden wind squalls, intense low-level wind shear, a rapid increase in the vertical displacement of air parcels, and a sharp pressure jump at the surface. In the front of the cloud, there is strong vertical motion that transports air up through the cloud and creates the rolling appearance, while the air in the middle and rear of the cloud becomes turbulent and sinks.

One of the most common type of wave clouds is known as the "undular bore".

According to Wikipedia: In meteorology, an undular bore is a wave disturbance in the Earth's atmosphere and can be seen through unique cloud formations. They normally occur within an area of the atmosphere which is stable in the low levels after an outflow boundary or a cold front moves through.

In hydraulics, an undular bore is a (weak) bore with a standing wave pattern at the downstream (subcritical) side.

A type of undular bore cloud is the "altocumulus" and the "altostratus undulatus" cloud.

The altostratus undulatus is a type of low altocumulus cloud with signature undulations within it. These undulations may be visible (usually as "wavy bases"), but frequently they are indiscernible to the naked eye. These formations will generally appear in the early stages of destabilizing return flows, especially over the southern plains of the United States, when the surface temperature is still relatively cool. The wavy strips of clouds are generally near an inversion surface.

Also referred to as billow clouds, wind row clouds, or wave clouds, variations of the undulatus can be elements that have merged or single elements that have stretched through the sky. They often run parallel, but can also appear to interweave across the sky, especially if dual wave systems are seen (also referred to as biundulatus). Gravity waves aligned normally to the direction of the wind can show wide, near-parallel cloud strips.

Here's a cool video of the Birmingham K-H wave clouds...