What is the effect of setting the altimeter to a lower than standard pressure?

When the altimeter is set to a pressure lower than the standard pressure of 29.92 inHg (1013.25 hPa), the instrument interprets this lower pressure as a higher altitude. This is because the altimeter measures atmospheric pressure and uses a standard reference altitude for its readings.

When the altimeter is set to a lower pressure, it interprets the current atmospheric conditions as being at a higher altitude than it actually is, causing the altimeter to show a decreased indicated altitude. Hence, if the altimeter is calibrated to a setting lower than that of the actual ambient pressure, it will indicate a lower altitude than what is true, resulting in the effect described.

True altitude remains unaffected because it is the actual height above sea level, which is determined by the current pressure at sea level, and not influenced by the way the altimeter is set. The downward adjustment in indicated altitude can lead to potential flight safety issues, such as flying lower than intended above ground level or encountering terrain hazards. Therefore, maintaining the correct altimeter setting relative to the pressure altitude ensures accurate readings and safer navigation.