ATMOSPHERIC WAVES 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.
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
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.