Why is it necessary for an altimeter to have a subscale adjustment?

An altimeter requires a subscale adjustment primarily to accommodate variations in atmospheric pressure that occur due to changes in weather conditions rather than altitude alone. When flying, the atmospheric pressure can change dramatically from the standard pressure levels, which can affect the accuracy of the altimeter readings. The aneroid capsule within the altimeter measures changes in air pressure to indicate altitude, but to ensure that it provides an accurate reading relative to the local pressure conditions, pilots need the ability to adjust the subscale.

By recalibrating the subscale to reflect the current local pressure, the altimeter can provide a more precise indication of actual altitude relative to sea level. This adjustment is critical for maintaining safety in navigation and landing operations, especially in environments where atmospheric pressure can be highly variable.