The new laser air speed system gets round these problems by bouncing light off air molecules and analysing the changes in reflection caused by the air’s motion relative to the aircraft.
It would seem likely that they are at a resonant frequency for one of the constituent bonds, likely N-N or O-O bond (N2 or O2) and thus able to get an absorbtion and re-radiation of the emitted energy. ie. a return that does not require dust, water droplets, or any other reflective constituent in the atmosphere. Pitot tubes-move over. Now we will just have to figure out how to keep the bugs off the "window".