Tuesday, August 30, 2005

Lenticular clouds

Last week I got two e-mails with photos from hiker friends who had hiked near Mt. Rainier in Washington. Carla H. hiked a segment of the Wonderland Trail, a 100-mile trail that goes around the base of the mountain. In July Skip D. attempted a trip to the summit that had to be aborted due to bad weather. But in Skip's e-mail he had a picture of lenticular clouds. I knew what they were by sight and even have a few photos of them from my Pacific Crest Trail hike last year, but didn't until now know what they were called. Then I brainstormed to figure out where I'd heard that term before. Ah hah. It was in some book my friend Shawn R. loaned me about UFOs and how lenticular clouds are often mistaken to be flying saucers.

len·tic·u·lar (l?n-t?k'y?-l?r) pronunciation
  1. Shaped like a biconvex lens.
  2. Of or relating to a lens.

[Latin lenticul?ris, lentil-shaped, from lenticula, lentil. See lentil.]

Where stable moist air flows over a mountain or a range of mountains, a series of large-scale standing waves may form on the downwind side. Lenticular clouds sometimes form at the crests of these waves.

Power pilots tend to avoid flying near lenticular clouds because of the turbulence of the rotor systems that accompany them, but sailplane pilots actively seek them out. This is because the systems of atmospheric standing waves that cause "lennies" (as they are sometimes familiarly called) also involve large vertical air movements, and the precise location of the rising air mass is fairly easy to predict from the orientation of the clouds. "Wave lift" of this kind is often very smooth and strong, and enables gliders to soar to remarkable altitudes. The current gliding world records for both distance and altitude were set using such lift.

Because these clouds have a characteristic lens appearance, or smooth saucer-like shape, they have been mistaken for UFOs (or "visual cover" for UFOs).

In the USA, lenticular clouds are relatively rare, but have been observed over the White Mountains of New Hampshire, over Mount Rainier, Washington, in the Rockies, and less often in Hawaii.