Lenticular clouds, also sometimes known as "lee clouds," "wave clouds" or by their scientific name of "altocumulus standing lenticular clouds," are smooth and shaped like lenses or lentils. They have been mistaken for UFOs because of their unusually uniform surface and saucer-like shape. Lenticular clouds may appear singly or in stacks, and form near tall geographic features that interrupt airflow and create standing waves in the atmosphere.
Lenticular clouds form when a tall geographic feature, such as the the top of a mountain, interrupts a strong wind coming up the side. The interruption in airflow creates a wind wave pattern in the atmosphere on the downwind side of the mountain. At the crests of these waves, moisture in the air condenses and forms a cloud, but as the air moves down into the trough of the wave the water evaporates again, leaving behind clouds in a characteristic lenticular shape.
Wave Wind Pattern
The wave wind pattern that creates lenticular clouds is analogous to the ripples caused by a rock jutting above the surface of a river. Just as ripples will appear in the same spot downstream from the rock while water continues to flow, lenticular clouds stay fairly stationary even though the air keeps moving past. The same sky may show cumulus or stratus clouds moving very quickly on strong winds while nearby lenticular clouds stay relatively still for hours. Airplane pilots know to steer clear of lenticular clouds to avoid turbulence, though hang gliders head straight for them to ride the air current for several miles.
Depending on the strength of the airflow and available moisture, the wave wind may yield several lenticular clouds piled on top of each other like plates. These stacks form due to stratification in the moisture levels in the air at high altitudes and usually contain no more than one or two clouds. Sometimes, however, spectacular formations of several clouds will appear and remain in the sky for hours.
Altitude & Climate
Lenticular clouds form at high altitudes of between 6,500 and 23,000 feet. They require a climate with stable, moist fast-moving winds in order to create the necessary wave wind in the atmosphere.
Lenticular clouds are most common in areas with high winds and tall mountains, and they are thus scientifically referred to as orographic formations from the Greek "oros," meaning "mountain." Lenticular clouds may also appear near elevated plateaus or high hills and occasionally near tall thunderclouds. They have been photographed on every continent in the world, including Antarctica.
- National Weather Service: Lenticular Cloud
- Wired: Weird, Rare Clouds and the Physics Behind Them
- Warren Wilson College, Physics Photo of the Week: Lenticular Cloud
- NOAA: Lenticular Cloud Formations, near Mauna Loa and Mauna Kea, November 25, 2003
- National Weather Service: Altocumulus Standing Lenticular
- Photo Credit shreckhorn peak with lenticular cloud image by Karen Hadley from Fotolia.com
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