Which of the following is a key characteristic of clear air turbulence?

Clear air turbulence is characterized by its occurrence in clear air, meaning it can happen without any visible warning signs such as clouds or storms. This type of turbulence is often found at high altitudes, typically around the jet stream, where warm and cold air masses meet. The absence of visual cues can make it particularly challenging for pilots, as it can be unexpected and difficult to predict.

The other options do not accurately define clear air turbulence: it is not restricted to low altitudes, nor is it primarily linked to thunderstorms, which tend to produce turbulence that is visible due to their associated cloud formations. Additionally, while it can affect any size aircraft, it is not exclusive to small aircraft and can impact larger commercial flights as well. Therefore, the defining characteristic of clear air turbulence is its ability to occur in clear conditions, illustrating its unpredictable nature.