What happens in higher altitudes to cause an aircraft to indicate a lower airspeed reading?

At higher altitudes, the density of the air decreases. This reduced air density means that fewer air molecules enter the aircraft's pitot tube. The pitot tube measures dynamic pressure, which is affected by the density of the air. As the air becomes less dense, the pitot tube receives fewer air molecules, leading to a lower indicated airspeed. Although the true airspeed may be consistent or even increasing, the aircraft's instruments reflect this lower reading due to the decrease in the amount of air impacting the pitot tube.

The other options do not accurately describe the relationship between altitude and indicated airspeed. For example, while temperature changes can affect air density, it is the decrease in the number of air molecules at higher altitudes that primarily causes the lower airspeed readings on instruments. Changes in fuel weight or the volume of air impacting the pitot are not the primary factors influencing indicated airspeed in the context of altitude and air density.