What atmospheric conditions are necessary for sky-wave propagation?

Sky-wave propagation relies on the behavior of radio waves as they interact with the ionosphere, a layer of the Earth's atmosphere that becomes ionized by solar radiation. For effective sky-wave communication, radio waves must be reflected back to the Earth by this ionosphere, allowing signals to travel beyond the horizon.

The correct choice highlights the essential role of ionospheric reflection, which occurs primarily during certain times of the day and under specific atmospheric and solar conditions. The ionosphere's ability to reflect radio waves is influenced by solar activity, including the density of ionization, which varies with the time of day and the level of solar emissions.

While the other options touch on aspects of weather or conditions affecting radio signals, they do not directly address the fundamental mechanism responsible for sky-wave propagation. For instance, clear skies and low humidity can affect signal clarity but are not prerequisites for sky-wave propagation itself. Similarly, high temperatures or strong solar activity can influence the ionosphere, but they are not the defining conditions needed for the waves to be effectively reflected, which is the core requirement for sky-wave communication. Thus, the emphasis on ionospheric reflection encapsulates the necessary atmospheric conditions for sky-wave propagation.