The longer wavelength of AM waves makes it possible to bend around a hill, whereas FM's shorter waves will be blocked.
Work Step by Step
As you can verify by looking on your radio dial, AM radio waves have a much smaller frequency and a much longer wavelength than FM radio waves. The amount by which waves bend (“diffract”) around an obstacle depends on size of the obstacle compared to the wavelength. Because the AM radio wavelength is large compared to the hill, AM radio waves readily bend around the hill.