What are clear air returns associated with?

Clear air returns are primarily associated with airborne targets that are not hydrometeors. This phenomenon occurs when radar systems detect objects that are present in the air but do not produce precipitation, such as insects, birds, or even man-made flying objects like aircraft. These returns can provide valuable information about the presence and movement of these non-meteorological objects, which is essential for applications such as aviation safety and wildlife management.

In the context of the other options, hydrometeors in precipitation typically produce radar returns that are distinctly different from clear air returns, as they reflect signals due to their water content and larger size when falling as rain, snow, or other precipitation types. Ground-based weather phenomena, while relevant to radar operations, are typically characterized by different return signatures that are not classified as clear air returns. Finally, severe weather events often involve intense precipitation and storm systems that generate significant radar echoes, contrasting with the low reflectivity associated with clear air conditions. Thus, the focus on airborne targets not related to precipitation makes the correct answer particularly relevant.