How do changes in air pressure affect indicated altitude on an altimeter?

When the air pressure decreases, the altimeter, which is calibrated to a standard atmospheric pressure, will interpret the lower pressure as being at a higher altitude than it actually is. This phenomenon occurs because the altimeter measures the atmospheric pressure and translates it into altitude based on a predetermined relationship between pressure and altitude under standard conditions.

In lower pressure situations, since the surrounding air has less pressure, the altimeter does not account for this change and indicates a higher altitude. Pilots often refer to this situation as an "over reading" because the altimeter suggests they are at a greater elevation than they truly are. This is crucial for navigation and safety, as misjudging altitude can lead to significant navigation errors, especially during approaches or in mountainous terrain.

Therefore, when air pressure is lower than the standard atmospheric pressure, it results in an over reading on the altimeter, confirming that the selection of lower pressure causing over reading is the correct interpretation in this context.