The AM broadcast band is very low in frequency. During the day, the ionosphere forms a "D" layer that absorbs low frequency radio waves. At night, the D layer is disappears and the AM broadcast band transmissions are then reflected off the ionosphere making long range reception possible.
So Mike, your assumption is true during the day, but not at night.
Another factor that effects what you can hear on the AM broadcast band via "skip" is the directivity of the antenna. If you are using an omnidirectional antenna, you are probably going to be hearing multiple stations on the same frequency. If you want to get serious, you need to find a directional antenna that will null out the stations you don't want to hear which in turn aims the antenna at the station you do want to hear. Here in the west, pretty much everything on AM is wiped out my Mexican stations. The AM broadcast band, along with pretty much all low frequency bands is strongly effected by man made and atmospheric noise. Getting the antenna up high gets it further away from made made noise sources. It also allows the antenna to work in the directional pattern it was designed for as the effects of ground are reduced, although this height would be very high for frequencies as low as the AM broadcast band.
The FM broadcast band is much higher in frequency. It is usually above the MUF (maximum usuable frequency) and goes through the ionosphere rather than being reflected by the ionosphere. This makes it good for line of sight (usually 10 degrees beyond the visual horizon) distances only, under normal conditons. Obviously the higher the antenna, the further the distance you can hear since this increases the distance to the horizon.
By the way, AM and FM are MODES of transmission. They are simply different methods of adding data to the carrier signal. They have nothing to do with frequency. However, in the US, we commonly use AM and FM to refer to two different frequency bands even though you could transmit either mode on either range of freqencies.
