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Mountain wave
clouds are produced by the winds at or near the ridgetops blowing perpendicular
to the mountain chain. For example, a wind from the west blowing across
the north-south oriented Cascade Mountains will at times produce mountain
wave clouds east of the mountains.
The wave clouds
can have various forms. Some of the more impressive forms are known
as Altocumulus Standing Lenticular, or ACSL. These often take the form
of pancakes, or even stacks of pancakes, and at times have been mistaken
for unusual aircraft.
On the 20th
of April, the weather pattern was rather unusual, with strong easterly
winds blowing over the Bitteroot Mountains of north Idaho. The resulting
ACSL clouds were a rare site for the Spokane area.
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www.weather.gov 
