What aspect does the airspeed indicator compare to indicate speed?

The airspeed indicator operates by comparing static pressure to ram pressure, which allows it to measure the aircraft's speed relative to the surrounding air (also known as indicated airspeed). When the aircraft moves through the air, the airflow creates a pressure differential, with ram pressure representing the dynamic pressure from the air entering the pitot tube, while static pressure is measured from static ports on the aircraft.

The difference between these two pressures indicates the amount of dynamic pressure produced by the aircraft's forward motion, which is then converted into a readable speed on the airspeed indicator. This is critical for pilot awareness as it helps in determining performance parameters like stall speed and maneuvering speeds.

The other choices involve comparisons that do not directly relate to how the airspeed indicator functions. Ram pressure is specifically tied to the dynamic air introduced when the aircraft is in motion, making the comparison with static pressure the only appropriate method for determining airspeed.