What is the primary cause of parasite drag in aircraft?

Parasite drag in aircraft is primarily caused by the various components on the aircraft's exterior, such as skin surfaces, antennas, and struts. This type of drag occurs when an aircraft moves through the air, creating resistance that is independent of the production of lift. Specifically, parasite drag increases with the speed of the aircraft and is influenced by the surface area and shape of these components.

The smoothness and design of the aircraft’s external features can significantly impact the amount of parasite drag encountered. For instance, protruding parts like antennas and struts disrupt the airflow, increasing turbulence around the aircraft and consequently the drag.

Other options mentioned relate to different aerodynamic forces or effects. Lift generated by the wings is associated with induced drag rather than parasite drag. Horizontal differences in air pressure might contribute to various aerodynamic phenomena but do not primarily cause parasite drag. If the Pitot tube opening were blocked, it would affect pressure readings and airspeed calculations but would not directly relate to the definition or cause of parasite drag itself.