that, considering the fact that wind velocities are a mere fraction of the speed of sound (750 miles per hour)? Dr. James B. Calvert, Associate Professor Emeritus of Engineering at the University of Denver, suggests that the phenomenon may derive from wind shears deflecting sound waves either downward (more toward the listener) or upward (away from earshot): When a wind blows, it is retarded at the surface -- a sort of boundary layer effect -- and increases in speed aloft. This is a wind shear. . . . A wavefront propagating with the wind will have its top inclined forward, so it will tend to return to the surface, while a wavefront propagating against the wind will be deflected upwards. ("Sound Waves," 2000) Naturally, if a unicorn sound is carried by the wind, the source of that sound will be upwind (opposite the direction of the gust). In the case of whirlwinds, anything goes. Accounts of unicorns' wind-swept voices in literature include: She speaks in a gentle voice like wind whispering through the trees. -- Rikale Fyrlight and Kalika Kiskmet, "The Pool" (2006) The wind carries their voices -- away fly the sentences like narrow ribbons. -- Katherine Mansfield, "The Wind Blows," Bliss, and Other Stories (1920)