Radio Mobile Coverage Chart
Radio Mobile is software for Windows (here running on Linux courtesy of WINE, the Windows Emulator), free for non-commercial use. It helps you visualize how VHF and UHF radio waves propagate between stations, and whether the signals are going to be strong enough to be copied at each end.
This is a coverage map superimposed on the (greyscale) topography. It is showing the received signal strength for a hypothetical mobile unit with specific receive characteristics, based on a base station at the center of the plot with specific transmission characteristics. In this case, it's roughly the coverage from an HT near my home to a similar HT roving the local area. Red means a strong signal, through blue meaning a weak signal.
Actual location of other stations and the strength of the communication does seem to follow this pattern pretty closely. It also lets you answer questions like, "what if I used a higher gain antenna another 6 meters higher off the ground?" and see the results of how the signals go over/around intervening obstacles.