Where is the worst CAT location in a high-level jetstream?

• A. Poleward side underneath jet core
• B. Equatorward side underneath jet core
• C. In the center of the jet core
• D. Above the jet core

Answer: (A)

Clear-air turbulence in a high-level jetstream is driven by strong wind shear and the way energy is organized around a jet streak. The most damaging turbulence tends to occur just below and to the north of the jet’s fastest winds—on the poleward side beneath the jet core. In that region, the air aloft surges ahead of slower air below and to the south, creating pronounced vertical and horizontal shear. The interaction of this shear with the jet’s curved flow creates dynamic instabilities that encourage small-scale turbulent motions, even where the air looks calm and there are no clouds.

To the equatorward side, the shear patterns are different and often less conducive to the strongest turbulence. In the center of the jet, the flow is more uniform and stable, despite the high speeds, so disturbances don’t grow as readily. Above the jet core, the atmosphere tends to be more stratified and less prone to the vertical motions that drive CAT, so turbulence is typically less severe there.