What happens to pressure altitude when atmospheric pressure decreases?

When atmospheric pressure decreases, pressure altitude increases. This relationship arises from the definition of pressure altitude, which is the height above the standard datum plane (a theoretical level where atmospheric pressure is 29.92 inHg or 1013.25 hPa). As atmospheric pressure drops, the pressure altimeter in an aircraft will read a higher altitude than what is physically present because it interprets the lower pressure at a higher elevation level.

In simpler terms, when you are at a given physical altitude and the outside air pressure decreases, the altimeter detects this change and indicates a higher altitude. This phenomenon is crucial for pilots to understand because it impacts aircraft performance decisions, navigational accuracy, and safety during flight operations.

The other choices suggest scenarios that do not accurately reflect the relationship between pressure and altitude based on atmospheric principles.