Which of the following is NOT one of the four turbulence intensity levels?

The four turbulence intensity levels that are commonly recognized in aviation are light, moderate, severe, and extreme. Each level describes the intensity of turbulence experienced in flight, affecting how the aircraft reacts and how comfortable passengers feel.

Light turbulence causes slight, erratic changes in altitude and attitude, with minimal effect on the aircraft's flight path. Moderate turbulence is more noticeable, with changes in altitude and attitude that may cause a feeling of unsteadiness. Severe turbulence is characterized by large, abrupt changes that can cause the aircraft to be momentarily displaced and may impose significant discomfort. Extreme turbulence represents a level of turbulence that can result in structural damage and severe disorientation.

The option that does not fit within these four standard categories is "heavy." While heavy turbulence might occasionally be colloquially used, it does not exist as an official turbulence intensity level in aviation terminology. Therefore, identifying "heavy" as not being one of the four turbulence levels is accurate.