What is indicated by an altimeter when flying in low pressure systems?

When flying in low pressure systems, the altimeter may indicate an altitude that is lower than the actual altitude. This occurs because altimeters are calibrated to standard atmospheric pressure at sea level, typically 29.92 inches of mercury (Hg). In a low pressure system, the actual atmospheric pressure is lower than this standard, which leads the altimeter to read a lower altitude than what the aircraft is truly at in relation to the ground.

Understanding this effect is crucial for pilots, as it emphasizes the importance of continuously monitoring weather conditions and adjusting altimeter settings accordingly. Failure to do so can result in unintentional flights at lower altitudes than intended, which could lead to safety issues, especially in areas with terrain.