When traveling from high pressure to low pressure, what happens to the altimeter reading?

When traveling from an area of high pressure to an area of low pressure, the altimeter reading will indicate a higher altitude than the actual altitude. This occurs because altimeters are calibrated to standard atmospheric pressure at sea level, which is 29.92 inches of mercury (Hg) or 1013.25 hPa.

As the airplane moves into a region of lower atmospheric pressure, the altimeter, based on its calibration, assumes that it is at a higher altitude than it actually is. Therefore, since the pressure is lower than the standard setting, the altimeter gives a reading that is higher than the true altitude. This phenomenon illustrates the importance of adjusting the altimeter to local pressure settings when flying, especially during descent or when entering different weather systems, to ensure accurate altitude readings.

In contrast, when moving into a high-pressure area, a calibrated altimeter would read lower than the actual altitude. The other possible scenarios do not apply as the altimeter does not remain constant or read actual altitude under these specific conditions when transitioning from high to low pressure.