In reality, radio propagation changes along with the weather and tropospheric ducting, and occasionally along with other upper-atmospheric phenomena like sunspots and even meteor showers.
Thus, while a broadcasting authority might fix the range to an area with exact boundaries (defined as a series of vectors), this is rarely if ever true.
ATSC digital television is affected by wind and trees (even if not surrounding the transmitter or receiver locations), apparently related to its use of 8VSB modulation instead of COFDM.
Satellite radio, which is designed for use without a dish, also uses ground repeaters in large cities due to the many obstructions their high-rise buildings cause to the many current and potential customers that are concentrated there.
Technologies are available that allow for switching to a different signal carrying the same radio program when leaving the broadcast range of a station.
HD Radio switches back to the analog signal as a fallback when the edge of the digital range is encountered, but the success of this from the listener's perspective depends on how well the station's broadcast engineer has synchronized the two.
The exact figure for various modes depends on how robust the signal is made to begin with, such as modulation, guard interval, and forward error correction.
In each of these three factors, the caveat is that a higher data signaling rate means a tradeoff with reduced broadcast range.
For HD Radio, the figure is only one percent of the station's analog wattage, in part because it is an in-band on-channel method, which uses sidebands that must prevent interference to adjacent channels, especially for older or cheaper receivers which have insufficient sensitivity and/or selectivity.