First of all, radar “picks up” a lot of things that are not solid objects, such as open radio signals (static) that are generated around the signal frequency the radar is operating at. This can be something as bilious as the open air between two clouds of relatively opposing charges, or a flash of lightning, neither of which involves solid objects -- other than charged atomic particles.

Anyway, as I said before:
"It will sometimes respond magnetically."

That means it will sometimes affect radar. Ball lightning has been shown to have an affect radio signals (produce static at the receiver).

Dr. Pyotr Kapitsa, co-winner of the 1978 Nobel Prize in Physics, believed that ball lightning occurred in regions where radio waves produced by thunderstorms interacted with each other to produce energy intensities high enough to convert ordinary air into glowing plasma.

(Source: Ball Lightning)

Well, radar works via a radio signal generator/receiver -- ergo...

Dan