Radio and TV signals aren't always limited to traveling line of sight distances. On HF this is commonly caused by refraction (bending) of the signal at the upper layers of the atmosphere known as the ionosphere. On frequencies higher than this, while solar activity plays a role, Earth based weather
events are the most common cause of propagation. These links explore several sites, and software,
that can be used to help predict how a HF signal will propagate. For propagation topics above 30 Mhz,
please see our DXing Above 30 Mhz article.
If you're just getting started in the HF hobby, AE4RV's Propagation Primer is an excellant place to start. Note that you need the most up to date Flash
software to view this site.