Text written by Mark Seltzer, all rights reserved.

In a stable atmosphere (where air wants to either sink or stay at the same altitude), you usually get zero convection. The weather is "flat", it doesn't want to go up. You need lots of "up" for thunderstorms. However, what it does invite into the photography realms is a nice display of fluid dynamics, in the form of gravity waves. For this, one needs a mechanism to force the air up such as a mountain range in a strong atmospheric flow.

Learning points
The Atmosphere behaves in exactly the same way as a fluid, and follows the laws of fluid dynamics. So waves in the ocean are akin to waves in the atmosphere.

If the gradient flow is strong enough in a stable atmosphere, it can force air upwards over a mountain range. When it descends on the other side, layers of atmosphere that have been pushed upwards fall back to their equilibrium point (as the air feels "uncomfortable", or out of balance, with the environment its been pushed into and wants to return back to its original altitude of equilibrium pressure. Away from the surface where there are no frictional forces, this can cause bounce-back as the vertical downward motion of the air again overshoots into an environment it's not conformable with, so the air rises again, and so forth. The result is a reoccurring wave.

The resulting wave is usually "standing", as the mountain range that produced it is not moving, so the waveform therefore doesn't move.

Stratiform clouds (flat stable atmospheric clouds) can form if the ascending air becomes saturated upon cooling, and you get Lenticular clouds, or other exotic forms of alto or cirro cloud sheets, giving away the presence of a standing atmospheric wave. The clouds will appear not to move as they ride the wave like a rollercoaster, forming on ascent, and melting on descent, when in fact the air is flowing really fast. Timelapse photography usually gives away this process.


Broad Standing Wave Example

Location: Exeter, Devon UK, morning of 23/02/2021


A very large conveyor belt of tropical maritime air, and associated cold front, was approaching the west of the UK bringing with it strong winds and stable atmosphere ahead of it. This created lee-waves downstream from hills and mountains, and in this case Dartmoor, Devon. This was a good example of when moisture condenses on the upward rise of a wave crest, making clouds on the spot. This particular atmosphere was also conducive to making turbulent stratocumulus, which gave asperitas/undulatus formations normally seen in altocumulus sheets during Spanish Plumes or fronts of mixed air masses. In the opposite direction to the camera was the majority of the wave trough (clear air) where drizzle from dissipating stratus was falling, apparently out of blue sky. It created a standing drizzle-bow which lasted for about half an hour.


Standing Lenticular Cloud Example

Location: Exeter, Devon UK, evening of 22/06/2020

  High level Lenticular clouds form over Dartmoor in a stable, yet dry southwesterly flow. A mixture of both standing and travelling waves look like ocean ripples. Was just asking for a timelapse and some ambient music.


Mark Seltzer  www.electricsky.co.uk


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