Words by Francesca Black ~ photos by Gavin John
Lenticular clouds (Altocumulus lenticularis) are seemingly stationary, lens-shaped clouds that form in the troposphere, usually aligning perpendicularly to the wind direction. Lenticular clouds can be sub-divided into altocumulus standing lenticularis, stratocumulus standing lenticularis and cirrocumulus standing lenticularis. These smooth, round or oval shaped clouds are characteristic of all high altitude mountain ranges and form in response to particular wind conditions. They are most often seen in the winter or spring when winds aloft are typically the strongest.
They are associated with waves in the atmosphere that develop when relatively stable, fast moving air is forced up and over a topographic barrier more or less perpendicular to the direction from which the upper-level wind is blowing, such as a mountain range. This deflection creates a gravity wave downwind of the topographic barrier similar to the waves generated by throwing a pebble into a pond. When the strong winds blow over the mountains and force moist air up to cooler elevations, the moisture condenses. As the winds blow back down the other side of the mountains, the moisture re-vaporises. When sufficient moisture is present above mountain-top level, lenticular clouds develop within the crest of these mountain waves where the air is rising.
The lenticular cloud is the condensed (visible) moisture under the wind stream. It doesn’t drift like other clouds because it is trapped in a pocket of relatively calm air just below the wind stream and just above the warmer air below. The notion that the clouds are stationary, however, is an optical effect. Lenticular clouds are continually forming at one end, in the vicinity of the wave’s crest and simultaneously dissipating at the other end, downwind of the crest. That is why they appear to remain in a fixed position even though winds are moving rapidly through the entire cloud.
Under certain conditions, long strings of lenticular clouds can develop, creating a formation known as a wave cloud. The distinctive shapes assumed by lenticular clouds, resembling a disc, series of stacked discs or a flat elongated form stretching parallel to the mountain range, has led to them commonly being described as “flying saucers” or “stacks of pancakes”. These striking shapes have caused lenticular clouds to be offered as an explanation for some Unidentified Flying Object (UFO) sightings.