Lenticular clouds are a favorite feature of Physics Photo of the Week. They show a remarkable simplicity, yet their dynamics are truly specular and somewhat unusual compared with "typical" clouds. They are called "lenticular" on account of their shapes like lenses - they also resemble a flying saucer. What is particularly interesting are the dynamics of these clouds.
The animation at right shows the interesting dynamics:
the clouds stay in one place in the atmosphere while the
wind and cloud particles move through them. The
"ordinary" low-level clouds move laterally with the
wind and pass the position of the lenticulars. The
lenticlar clouds stay put at a much higher elevation.
If you watch an individual "cloudlet" in the lenticular
formation, it tends to follow an arc - it briefly rises
through the cloud then smoothly descends and evaporates on
the downwind side of the cloud. These clouds tell us
that the air is moving in a wave up-and-down. The wave
is stationary - the wave crest stays in the same place -
similar to the ripples in a river downstream from a partial
obstruction. In this case, the wave may have been
formed downstream from the main chain in the Green Mountains
in Vermont - about 30 miles west of this formation. As
the moist air rises, it expands, cools, and the moisture
condenses into cloud. As the air parcel descends on
the downwind portion of the cloud, it is compressed by the
rise in pressure, warms up, and the cloud evaporates and
Lenticular clouds are somewhat rare because the
atmosphere must be extremely stable. Usually the
atmosphere is unstable. In a normal atmosphere, as air
ascends expanding and cooling, it is still more buoyant
compared to the higher-level surrounding, and it continues
to rise. This is a positive feedback creating a
"runaway" effect. In a stable atmosphere, the air at
higher levels is warmer than normal, less dense, and the
lower density of the surrounding air results in a lesser
buoyancy for the risen air, so it returns to its original
elevation. This is a thermal inversion - resulting in
Even though lenticular clouds are my favorite type of
cloud formation, the last image was published on September
10, 2010 - almost four years ago. A diagram of
the atmospheric airflow is included in PPOW
for March 10, 2006.
Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature interesting phenomena in the world around us. Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to email@example.com.
All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion. These photos and discussions may be used for private individual use or educational use. Any commercial use without written permission of the photoprovider is forbidden.