The ground wave
The ground wave travels along the surface and is weakened by the soil. Salt water conducts well, so signals go much further over the sea than over dry ground or rock. Power, a good vertical antenna and a good earth system are what count at these frequencies. Typical ranges are of the order of a hundred kilometres or more by day, depending on power, soil and the noise at the receiver; these are rough figures.
Night and the sky wave
In daytime the D layer absorbs medium waves that go up. At night it fades and the signal is returned from the E and F layers, so distant stations appear on the same dial, sometimes from other countries, and they interfere with the local ones. This is why broadcasters change power or pattern at night and why the evening reception on 160 m is so different from the day.
Try it on the map
Choose a frequency below 1.6 MHz on the coverage map: the engine switches to the ground wave model and, at night, adds the sky wave. Change the ground type to see how the soil changes the range. For the physics of the other bands read radio frequency bands from 0 to 3000 MHz.
