Here's why:

* The Ionosphere: This layer of the Earth's atmosphere is located about 50-400 miles above the surface. It contains electrically charged particles (ions) which can reflect radio waves back down to Earth.

* Reflection: The interaction between the radio waves and the ions in the ionosphere causes the waves to change direction, essentially bouncing them back towards the ground. This is why AM radio stations can be heard over long distances, as their signals can "skip" off the ionosphere and reach listeners far beyond the line of sight.

Important Note: The reflection properties of the ionosphere vary depending on factors like the time of day, season, and solar activity. This is why shortwave radio reception can be affected by these factors.