Why Maximum Usable Frequency is the highest frequency that will still refract back to Earth over a given path, so it depends on both the ionization level of the ionosphere and the geometry of the path. Ionization is driven by solar radiation, which varies with time of day, season and the solar cycle, and can be upset by flares and geomagnetic storms. Path distance and location matter because a longer hop strikes the ionosphere at a shallower angle, which refracts higher frequencies back down, while a short, steep path has a much lower MUF. Since all of these change the MUF, every listed factor applies.