Air currents are detectable a certain power outputs.
Certain IR jamming equipment under development at this moment heats up air arond the aircraft, creating odd hot spots.
Sometimes the disturbance itself from the heating is strong enough to be detected.
One way to detect a stealth aircraft is to detect the wake turbulance that it leaves behind it.
That is possible regardless of moisture content, but whether or not it is reliable or the info can be filtered enough for use is another story.
When radar starts detecting air disturbances, it detects quite alot of stuff, so filtering the info gathered and paring it down to useful info is difficult at best.
OK, that makes sense, kind of, that the turbulence caused by a plane might be detected, what with temperature differences.
But I am assuming that radar cannot detect clear air turbulence that has no machinery associated with it.