What is defined as a zone of silence between the point where the ground wave becomes too weak for reception and the point where the sky wave is first returned to earth?

The concept described in the question refers to the "Skip Zone," which is a specific area where signals from a radio transmitter are not detectable due to the interference of ground waves and sky waves. Ground waves travel along the earth's surface and can become too weak for reception at a certain distance from the transmitter. Meanwhile, sky waves are reflected back to the earth by the ionosphere, but this reflection does not occur until the waves reach a certain distance.

The space where reception fails due to the ground wave's weakness and before the sky wave returns is known as the Skip Zone. This phenomenon is particularly relevant in the context of high-frequency (HF) radio communications, where the behavior of radio waves is influenced by ionospheric conditions.

On the other hand, the terms "Temperate Zone," "Torrid Zone," and "Frigid Zone" refer to geographical climate zones defined by temperature ranges and do not relate to radio wave propagation or communication principles. Thus, they are not relevant to the question.