What does the vertical speed indicator (VSI) compare to show the rate of descent or climb?

The vertical speed indicator (VSI) functions based on changes in static pressure to determine the rate of descent or climb. The VSI is connected to the static pressure system in the aircraft and measures the differences in static pressure as the aircraft ascends or descends. As the plane climbs, the static pressure decreases, and this change is detected by the VSI mechanism. Conversely, during a descent, the static pressure increases.

The calibrated leak within the VSI allows the instrument to respond to these pressure changes over time, providing a relatively smooth indication of vertical speed. This process enables the pilot to monitor whether the aircraft is climbing, descending, or in level flight, helping them maintain desired flight profiles.

Other options do not accurately describe the principles behind how the VSI functions. The relationship between temperature and altitude is more relevant to gauge performance and density altitude calculations rather than direct vertical speed indication. Static pressure and flow rate pertain to different aspects of airspeed and altitude measurement rather than vertical speed. Similarly, ram pressure is related to dynamic pressure changes, significantly affecting other instruments but not the VSI's operation regarding ascent or descent rates.