After the rain, Kelvin-Helmholtz clouds spotted in Morgan County

After a good, soaking rain, the clouds can produce interesting features, including the ones spotted Tuesday afternoon over Morgan County by David Graves (above).

Two such features are gravity waves and Kelvin-Helmholtz clouds, as noted by Chief Meteorologist Jason Simpson in his tweet of David’s photo.

Gravity Waves

Gravity waves happen when a disturbance in the atmosphere causes air to vacillate, or waver, in an up-and-down motion. The rising air condenses and the water vapor within it cools, producing streets or rows of clouds.

Kelvin-Helmholtz Instability

When we think of the word “fluids”, we often think of liquids, like water. The reality, though, is that the atmosphere (which is made of a mixture of gases) also acts like a fluid. Within the atmosphere, different air masses of varying densities and speeds are interacting and mixing with each other.

In the case of Kelvin-Helmholtz waves, air closer towards the ground is moving more slowly compared to air located higher aloft. This difference in air, or wind speed, is called vertical shear, and it causes the top portion of the cloud to curl under — like an ocean wave curling and crashing upon itself.

Kelvin-Helmholtz waves are named after  Lord Kelvin and Hermann von Helmholtz, who discovered the fluid dynamical processes that cause the waves.