Extremely rare clouds that look like ocean waves photographed over Virginia mountains

When wind conditions are just right, cloud formations can be whipped up to look like ocean waves -- a rare phenomenon captured by photographer Amy Hunter at Smith Mountain Lake in western Virginia on Tuesday.

These wave-like clouds are called Kelvin-Helmholtz clouds, a nod to Lord William Kelvin and Hermann von Helmholtz, the meteorologists whose interest in turbulent airflow led them to study the physics behind this type of formation.

Also referred to as shear-gravity clouds or billow clouds, these formations most commonly occur on windy days when there is a difference in the speed, direction, or density between two wind currents at different levels in the atmosphere. When wind moves faster at the top of the cloud formation than at the bottom, the faster-moving air can whip up curls of the cloud formation from the slower-moving level of air below, causing the wave-like clouds.

Kelvin-Helmholtz instability doesn't solely affect clouds; it can occur wherever there are two fluids moving at different velocities while interacting with each other. It's the same phenomenon that creates waves on water as air blows over the surface of a body of water.

Turbulence for aircraft can also be attributed to Kelvin-Helmholtz instability.

Kelvin-Helmholtz clouds are so rarely seen because they are often hidden behind larger areas of cloud coverage, or there are no clouds in the sky to illustrate the underlying air patterns.