zone of silence
- The region above an altitude of about 160 kilometers from the earth's surface, where the distance between air molecules is greater than the wavelength of sound, thus sound waves can not be propagated. This zone extends well beyond the planet into outer space. Also called anacoustic zone.
- A zone which runs from where ground waves become too weak for reception to the point where a sky wave returns to the surface of the earth. Also called skip zone, or silent zone.