What occurs when flying from an area of high pressure to low pressure?

When transitioning from an area of high pressure to low pressure while flying, the altimeter, which is calibrated to a standard atmospheric pressure, will indicate a higher altitude than the actual altitude being flown. This is because the atmosphere is less dense in low pressure areas. As aircraft move from high to low pressure, the altimeter settings remain based on the standard atmospheric pressure at sea level, which can result in an indication that is higher than the true altitude due to the lower pressure conditions of the environment.

Pilots must be aware of this phenomenon as it is crucial for maintaining safe altitude awareness during flight. If the pilot does not update the altimeter setting to correspond with the local pressure conditions, the aircraft could potentially be flying lower than indicated, increasing the risk of terrain or obstacle collisions.