When moving from low pressure to high pressure, what does the altimeter read?

When flying from a low-pressure area to a high-pressure area, the altimeter, which is calibrated to atmospheric pressure, will read lower than the actual altitude. This occurs because the altimeter detects the increase in atmospheric pressure associated with moving into a high-pressure zone.

In a low-pressure environment, the air is less dense, and the altimeter is set such that it reflects lower altitudes. As the aircraft transitions into a high-pressure area, the air density increases, resulting in higher actual terrain elevation compared to the altitude reading provided by the altimeter. This phenomenon is crucial for pilots to understand, as failing to adjust the altimeter correctly can lead to altitude miscalculations, potentially resulting in flight safety issues.

The significance lies in the necessity for accurate altitude readings during flight operations, making it essential for pilots to regularly verify and adjust their altimeter settings to account for changes in atmospheric pressure to maintain safe altitude levels.