Altocumulus lenticularis clouds are named for their smooth, round, lens-like shape (lenticular means lens or lentil-like). These middle-level clouds can form spectacular patterns that delight weather-watchers and photographers and have almost certainly been responsible for a number of UFO sightings over the years.

When wind blows across a mountain range, it tends to form air waves on the lee side of the mountains. This process, known as the mountain wave effect, is usually invisible, but when moisture is present at the top of these waves, lenticular clouds form where the wind rises and dissipate where it falls.

Because mountain ranges are nearly always of irregular shape and wind may move at different speeds at different levels, the waves produced in this manner often have varying distances between their crests (referred to as the wavelength) and the resulting clouds form an irregular pattern.

However, if the mountain range has a fairly regular shape and the wind is blowing at a steady speed at approximately right angles to the mountains, the wave crests, and any resulting clouds, will form a regular pattern. Furthermore, if alternate layers of moist and dry air are present above the mountains, the clouds may pile up on top of each other like stacks of plates. It is these distinctive stack formations that have, on occasion, been mistaken for UFOs.

If the wind generating the waves has a fairly constant speed, the cloud pattern will be stable and long lasting, remaining virtually stationary in the sky for extended periods. Generally, no significant weather is produced by altocumulus lenticularis, but, occasionally, if there is sufficient moisture in the surrounding atmosphere, these clouds can become thick enough to generate light rain, or snow showers in sub-zero temperatures. Because these formations are associated with high-speed winds in the middle layers, they may be precursors of windy conditions at ground level.

Surfing The Waves

The roller-coaster motion of the air, made visible by the formation of these clouds, can produce significant levels of turbulence, and commercial airliners will try to avoid such areas. High-level glider pilots, however, sometimes do the opposite: they look for lenticular clouds as signs of a source of uplift for the aircraft. The gliders can surf along the mountain waves, maintaining altitude by remaining on the rising side of the wave crest.

Lenticular clouds occur in most parts of the world, and may form over quite small mountains. Good examples occur when moist winds blow in off the Pacific Ocean and encounter the Sierra Nevada in California. However, the most dramatic clouds are generated by the largest ranges, such as the Himalayas, Andes, and Rocky Mountains.