by Skip Via
skip@westvalleynaturalists.org
Click on images to enlarge them.
If you were in the West Valley area during the middle of the afternoon of December 9, 2025 and happened to be looking through The Notch toward GNP , you may have seen a rare and typically brief atmospheric phenomenon–a Kelvin-Helmholtz cloud system forming.
Kelvin-Helmholtz clouds look exactly like waves breaking in the ocean. That’s because their formation is the result of the physics of fluid dynamics–specifically, the Kelvin-Helmholtz Instability.
The Kelvin–Helmholtz instability (after Lord Kelvin and Hermann von Helmholtz) is a fluid instability that occurs when there is velocity shear in a single continuous fluid or a velocity difference across the interface between two fluids.
https://en.wikipedia.org/wiki/Kelvin–Helmholtz_instability
If you’ve seen images of Jupiter–the Red Spot and other surface storms–you’ve seen an example of the KH instability as high surface winds cause shearing between masses of gases of varying densities moving against each other at different speeds.
That is exactly what was happening over the mountains of GNP on December 9th. The phenomenon pictured in this article occurred as a large atmospheric river (the “Pineapple Express”) was moving into the valley. The cooler, denser, mostly stagnant air in the valleys was being overridden by a fast-moving stream of warmer, less dense mass moving at a significantly higher speed. The shear forces between the two air masses stirred up the waves that you’re seeing in these photos.
I watched the progress of the waves for probably 5-6 minutes. They were clearly moving and changing rapidly as the air in the masses mixed and the waves became less distinct. After 10 minutes, there were no significant waves to be seen.
KH instability is not limited to air masses nor does it require clouds. The same principle can happen when two liquids of different densities slide past each other, and the phenomenon can be measured in clear air as well. The only conditions required are two air masses of different density moving against each other.
Pilots avoid KH clouds when possible, as the air along their formation is turbulent.
Kelvin-Helmholtz clouds are rare, at least in part because they tend to be visible only briefly.
If anyone has any additional photos to share of this event, please consider sending them along and I’ll add them to this post–with full credit, of course.
