cumulo-granite
n. Pilot slang for mountains or cliffs hidden by clouds.

Example Citations:
“My art has no object, no image, no point of focus.” I remembered James Turrell’s words as we flew through the clouds and between the mountains. The electric grid of Phoenix glowed in the Arizona night, then the plane climbed to the high plateau of Flagstaff and dipped to the tiny airstrip. Instrument flying, when you see nothing but cloud beyond the cabin, requires great skill, Turrell later explained, or you might hit “cumulo-granite”.
—Mark Holborn, “Under the Volcano.” The Independent, April 11, 1993

Those high mountains, often cloud-covered, were unforgiving of lapses in attention to the details of navigation. We occasionally came back from a mission to find bunks left empty after a pilot encountered “cumulo-granite” because he misunderstood radio information and started his descent before it was safe to do so.
—Robert K. Watts, “Letter to the Editor,” The Bismarck Tribune, October 14, 2002

Earliest Citation:
Flying huge aircraft capable of carrying 400 passengers is a dangerous occupation, requiring a combination of flying skill and mental agility. Make a slip, fail to detect a mistake by air-traffic controllers and you could find yourself flying into “cumulo-granite”, to borrow a popular pilots’ expression.
—Michael Harvey, “Stress in the Air: A pilot’s life,” Courier-Mail, March 8, 1989

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