What happens to the altitude indication if air pressure decreases?

When air pressure decreases, the indicated altitude on an altimeter increases. This occurs because the altimeter is a barometric device that measures the weight of the air above it. An altimeter is calibrated to read the atmospheric pressure at a specific altitude under standard conditions. As the air pressure decreases—possibly due to gaining altitude or weather changes—the altimeter interprets this reduced pressure as a higher altitude, causing the indicated altitude to increase.

In practical terms, when a pilot is climbing, the actual altitude is increasing but the pressure is decreasing due to the less dense air at higher elevations. If a pilot were to maintain the same setting on the altimeter and ascend into an area of lower pressure, the altimeter would continue to indicate a higher altitude than the aircraft is actually at. Thus, a decrease in air pressure directly correlates with an increase in the indicated altitude on the altimeter.