What does a sudden change in wind velocity indicate?

A sudden change in wind velocity is indicative of windshear, which refers to a rapid change in wind speed or direction over a short distance in the atmosphere. Windshear can occur at various altitudes and is particularly significant during takeoff and landing phases of flight, as it can affect the aircraft's performance and control. Recognizing windshear is crucial for flight safety because it can lead to unexpected changes in an aircraft's altitude and airspeed, potentially resulting in dangerous situations such as loss of control.

While severe turbulence may also be associated with wind changes, it typically refers to the intensity of air movement rather than a specific change in wind velocity. Calm weather and clear skies suggest a stable atmospheric condition without the sudden shifts in wind that characterize windshear. Therefore, the most accurate answer regarding a sudden change in wind velocity is windshear.