What is a likely location of clear air turbulent conditions?

In the context of clear air turbulence, the correct choice points to the area in and around an upper trough on the polar side of a jet stream. Clear air turbulence is typically encountered when there are strong vertical winds or changes in wind direction.

An upper trough on the polar side of a jet stream is characterized by significant wind shear due to the strong lateral gradients of wind speed. This wind shear occurs as the fast-moving airflow of the jet stream interacts with the slower-moving air on the polar side, creating turbulent conditions. Pilots and dispatchers need to understand that clear air turbulence is most common in these scenarios, as the interactions between differing air masses can cause sudden and unexpected turbulence, even in clear skies without visible clouds.

In contrast, other options do not align with the typical conditions for clear air turbulence. For instance, ridges associated with high-pressure systems can often lead to more stable air conditions, while downstream areas of a jet stream can also be less turbulent under certain circumstances. Similarly, turbulence over land masses during summer is generally more related to thermal activity and convective phenomena rather than the clear air turbulence mechanisms present in the upper trough scenarios.