Planes cast defined shadows when at or below around 4000ft. The shadow seems to disappear somewhat as the aircraft gets higher or if the aircraft is smaller.
Shadows are one way I determine my position in relation to other aircraft when flying a traffic circuit (real world).
Interestingly, when you look at your own shadow during (real) flight, there is a lovely circular halo of light around it, that is accented when the shadow falls onto clouds.
