Courtesy of Klems data tool and his work it has been established that the basic CLOD atmospheric temperature lapse rate is 1.97 deg C per 1000feet (corrected to 2 decimal places). This has now been measured up to 26,000feet. So its only 0.01 deg C per 1000 feet out ... not much and well and truly in the ball park to the ISA value of 1.98 deg C per 1000feet.
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