How is altitude expressed in the standard pressure region of aviation?

In aviation, altitude in the standard pressure region is expressed in feet above sea level (ASL) along with flight levels. This is important as it provides a clear and standardized way of communicating altitude information to ensure safety and efficiency in air traffic control and navigation.

Flight levels are used for aircraft flying at and above the transition altitude, where the pressure setting of the standard atmosphere is used (29.92 inches of mercury). When pilots communicate their altitude in feet ASL below this transition altitude, it is to account for variable atmospheric pressures affecting true altitude. Above this altitude, flight levels represent a uniform reference that optimizes separation between aircraft, irrespective of local barometric pressure changes.

Thus, the combination of expressing altitude in feet ASL and using flight levels encapsulates the need for both precision in communication and adherence to standard practice in aviation altitude reporting.