What happens to the altimeter reading when transitioning from high pressure to low pressure?

When transitioning from high pressure to low pressure, the altimeter typically indicates a lower altitude than the actual altitude. This occurs because altimeters are calibrated to report altitude based on the standard atmospheric pressure setting. When the atmospheric pressure drops (as is the case when moving from high to low pressure), the altimeter does not automatically adjust for this change in pressure unless it has been manually reset to the new local altimeter setting.

Thus, if a pilot is flying in high pressure conditions and then descends into a low pressure area without adjusting the altimeter, the instrument will still indicate the altitude based on the higher pressure setting, leading to a false reading of being lower than the actual altitude. This discrepancy can create dangerous situations for pilots, as they may unknowingly be flying at a higher altitude than what the altimeter indicates, increasing the risk of obstacles such as terrain or obstacles during flight.