What describes a sudden change in wind direction or speed over a short distance?

The correct choice refers to wind shear, which is characterized by a sudden change in wind direction or speed over a short distance in both vertical and horizontal planes. This phenomenon can occur at any altitude and is particularly important during takeoff and landing phases of flight, as it can significantly affect aircraft performance and safety.

Wind shear can arise from various atmospheric phenomena, including temperature inversions, frontal boundaries, or turbulence caused by terrain. Pilots must be trained to recognize and respond to wind shear to maintain safe flight operations, as it can lead to abrupt altitude changes or changes in aircraft attitude that demand immediate corrective action.

The other options present different concepts related to aerodynamics and flight. Crosswind refers to wind that blows across the runway, impacting takeoff and landing but does not signify a sudden change in wind characteristics over short distances. A microburst is a specific type of severe downdraft characterized by intense, localized wind, usually associated with thunderstorms, which can also indicate sudden changes but is more about downdrafts than horizontal shifts. Clear air turbulence describes turbulence occurring in clear air, usually associated with jet streams but does not specifically relate to the rapid change in wind conditions.