Long distance radio broadcasts use short wave bands. Why?

Shortwave frequencies are capable of reaching any location on the Earth because they can be reflected by the ionosphere (a phenomenon known as Skywave propagation). The selection of a frequency to use to reach a target area depends on several factors:

- The distance from the transmitter to the target receiver.
- Time of day. During the day, frequencies higher than approximately 12 MHz can travel longer distances than lower ones; at night, this property is reversed. The dependence on the time of the day is due to a particular transient atmosphere ionized layer known as the D Layer, forming only during day when photons from the sun break up atoms into ions and free electrons. This layer is responsible for partial or total absorption of particular frequencies.
- Season. During the winter months the AM broadcast band tends to be more favorable because of longer hours of darkness.
- Solar activity. Sunspots, solar flares, and overall solar variation affect the ionosphere. Solar flares can prevent the ionosphere from reflecting or refracting radio waves.