When flying from higher pressure to lower pressure, what does the altimeter indicate?

When flying from an area of higher pressure to an area of lower pressure, the altimeter, which is calibrated based on sea level pressure, will indicate a higher altitude than the aircraft's actual altitude. This occurs because the altimeter measures the atmospheric pressure and uses it to determine altitude; as the aircraft moves into an area of lower pressure, it senses a drop in pressure and interprets this as a rise in altitude, causing the instrument to display a higher altitude reading than what is accurate.

Therefore, the altimeter can give a misleading indication of altitude unless adjustments are made for changes in the local barometric pressure. Pilots must be aware of this potential discrepancy, especially during descent through various pressure systems, to ensure safe altitude maintenance and adherence to altitude assignments from air traffic control.