At what temperature can outside air at 18,000 feet be expected to be?

At an altitude of 18,000 feet, the temperature typically decreases with altitude in the lower levels of the atmosphere, generally following the standard lapse rate, which is about 2°C per 1,000 feet. At sea level, the average temperature is around 15°C. If you apply the lapse rate to calculate the temperature at 18,000 feet, you would expect a decrease of approximately 36°C (18,000 feet × 2°C/1,000 feet).

Starting from 15°C at sea level and subtracting 36°C gives a result of around -21°C, which closely aligns with -20°C from the choices presented. Therefore, selecting -20°C is a reasonable expectation based on this understanding of the atmospheric temperature profile with altitude.