Under which condition is clear air turbulence (CAT) most likely to be encountered?

Clear air turbulence (CAT) is most commonly associated with significant wind shear in the absence of any visible weather phenomena, such as thunderstorms. Among the conditions listed, the scenario where constant pressure charts show isotachs (lines of equal wind speeds) less than 150 nautical miles apart indicates a steep gradient of wind speed, which is indicative of wind shear.

When isotachs are close together, it typically represents a rapid change in wind speed over a short horizontal distance, creating unstable airflow that can lead to the presence of CAT. This phenomenon is particularly common at high altitudes, especially near the jet stream, where strong wind patterns are prevalent.

In contrast, the other scenarios do not directly suggest the conditions necessary for the formation of clear air turbulence. A slow-moving sharp trough might lead to some turbulence but is more likely to produce turbulent conditions in the presence of storm systems. High-speed wind patterns that are consistent may not necessarily indicate turbulence without the presence of significant wind shear. Additionally, a lack of temperature changes does not contribute to the factors typically leading to CAT, as temperature differentials can be significant in causing turbulence. Hence, the condition indicated by closely spaced isotachs is the most directly related to the likelihood of encountering clear air turbulence.