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You have no doubt seen them in the sky this winter, perhaps
snapped a photo or two and used terms like
"flying saucer" or "stack of pancakes" to
describe them. So what
exactly are those mysterious looking clouds and why do they
form?
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Photos above (Albuquerque) and
below (west of Carlsbad) were taken
the same day: January 18, 2006
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Known as Altocumulus Standing Lenticular (ACSL) or
Altocumulus Standing Lenticularis clouds, they are associated with waves
in the atmosphere that develop when relatively stable, fast moving air
is forced up and over a topographic barrier that is oriented more or less perpendicular to
the direction from which the upper-level wind is blowing. This deflection creates a
gravity wave downwind of the topographic barrier not unlike a wave
you might generate by throwing a pebble into a pond. When sufficient
moisture is present above mountain-top level, ACSL clouds develop
within the crest of
these mountain waves where the air is rising. ACSL clouds are continually developing and dissipating in the vicinity of the wave's crest and
immediately downwind of the crest, respectively. That is why they appear to
remain stationary (hence the name) even though winds are swiftly
(sometimes very swiftly) moving through the
entire cloud. They are most often seen in the winter or spring
when winds aloft are typically the strongest.
A
more thorough discussion of mountain waves is available here.
When aircraft encounter a mountain wave, severe turbulence is
often the result. The most telling clue that a mountain wave
exists is the presence of ACSL clouds. But when the air is too
dry to support cloud development, pilots and others may be
unaware that a mountain wave and potentially severe turbulence
exists. So while visually striking and appealing to photographers,
ACSL may not be a welcome sight to those in the aviation
community. Additionally, strong and gusty surface winds may be
encountered particularly in the lee of the mountain
barrier.
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Pictured
below are ACSL clouds over the Gila
Mountains near Glenwood in April 2005.
Dominated by periods of strong, mostly dry northwest winds aloft,
the upper-air pattern so far during the winter of 2005-06 has been down right dismal
for precipitation across New Mexico. It has favored among all things
mountain wave development and increased windiness. The image
below depicts the departure from normal of wind
speed (m/s) at roughly the 10,000 MSL level between December 1, 2005
and January 31, 2006. Note the shades of orange and red, which indicate
areas where free-air wind speeds were above the long- term
average.
Departure from normal mean wind speed
(m/s) at roughly 10K feet MSL between
December 1, 2005 and January 31, 2006
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The image above depicts the Albuquerque upper-air
sounding valid at 5am MST February 1, 2006. Note the
inversion, or shallow layer of stable air at ~2km MSL
just above the level of New Mexico's high terrain. Northwest
winds of 30-40 knots were also present through a deep layer
of the lower troposphere. This sounding was
taken the
day (within a couple hours) that an impressive ACSL cloud
was captured on film just north of Gallup and is typical of
days when ACSL clouds are present.
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weather.gov 
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