What are sky waves?

Sky waves refer specifically to radio waves that are reflected off the ionosphere. This phenomenon occurs when radio waves are transmitted at a certain frequency that allows them to interact with the ionosphere, a layer of the Earth's atmosphere that is ionized by solar radiation. The ionosphere can reflect certain frequencies of radio waves back to the Earth's surface, allowing for long-distance communication. This is particularly useful for frequencies in the high-frequency (HF) range, typically between 3 to 30 MHz, as they can travel beyond the horizon and connect distant locations.

The other choices do not accurately describe sky waves. For instance, while radio waves that travel in straight lines do exist, they describe ground waves and line-of-sight communication, not sky waves. Low and high power radio waves refer to the strength of the signal rather than the propagation method, which is the key characteristic of sky waves.