In aeronautics, the relative wind is the direction of airflow over the airfoil, i.e. in vector terms it is the vector sum of the ambient air past the aircraft: Most usefully, it is the direction of the air over the aircraft's wings and control surfaces. The angle between the wing chord line and the relative wind defines the angle of attack for an airfoil. Note that the relative wind can be in any direction at any velocity, and not necessarily parallel to the surface. The relative wind is of great importance to pilots because exceeding the critical angle of attack will result in a stall, regardless of airspeed.
Relative wind is also used to describe the airflow relative to one's body during freefall i.e. during a skydive or BASE jump. In a normal skydive the vertical descent of the skydiver creates an upward relative wind. The relative wind strength increases with increased descent rate.
The relative wind is directly opposite to the direction of travel.
Therefore, when a skydiver exits a forward-moving aircraft such as an aeroplane, the relative wind eminates from the direction the aeroplane is facing due to the skydiver's initial forward (horizontal ) momentum. As gravity gradually overcomes this forward momentum and attracts the skydiver downward, the relative wind alters proportionally into an upward (vertical ) direction. This creates an arc of travel for the skydiver similar to water flowing from a low pressure hose held horizontally and creates a variation in the pitch of the relative wind from horizontal to vertical.
When exiting the plane during a normal belly-to-earth skydive, the skydiver must arch his body in the direction of travel which is initially horizontal. If the skydiver continues to arch, his belly will gradually alter pitch until he is belly-to-earth. This section of the jump is commonly referred to as "the hill"
Relative wind differs from the wind we refer to in meteorology in that the object (e.g. the skydiver ) moves past the air, as opposed to the air moving past the object.