Comment by superkuh
FM radio is line of sight because of it's high frequency. Because of AM's much longer wavelength it can bounce off the ionosphere and it drops off slower with distance than FM. The range of AM stations is much greater than a comparable FM station for a given height-above-terrain of the antenna.
AM waves can also penetrate and pass through objects a lot better thanks to those lower frequencies.
Driving at night in the intercity mountain-west US, you can pick up radio stations quite far away. Not too long ago outside of Boise I was able to pick up some AM stations from Cleveland!