Diffraction effects (the bending of waves around the edge of an object) are evident only when the size of the wavelength of the wave is on the order of the size of the object (or larger). Buildings and hills are much larger than FM waves, so FM waves will not diffract around the buildings and hills. On the other hand, these objects are smaller than AM waves, so the AM waves will diffract around them easily. The AM signal can be received behind the objects.