Physics Photo of the Week

August 29, 2014

Lenticular Clouds
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 laterallyLarge
              file loading - please be patient 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 disappears.

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 stable atmospheres.

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 dcollins@warren-wilson.edu.

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