How far such a signal goes is dependent on a number of factors, principally transmitter power, operating frequency and earth conductivity. Groundwave propagation is heavily dependent on the frequency, with low frequency signals travelling greater distances. In fact, every thing else being equal, groundwave signals from a station on 550kHz will travel twice as far over land as those radiated by a station on 1500kHz. The earth conductivity is also a very significant factor and it is found that the better the conductivity the further the signal travels. In contrast, a similar signal travelling over rocky terrain would carry only about one quarter of this distance.
Groundwave propagation is very stable resulting in consistent reception conditions.
Because of its stable daytime behaviour, radio stations usually optimise their aerials to radiate as much of their signal as possible via the groundwave in order to improve coverage. There exists a rarefied region of the earth's upper atmosphere that absorbs the intense solar ultra-violet radiation thereby protecting life on the earth's surface. These layers have a profound effect upon radio waves approaching them from transmitters on the ground below. Under certain conditions refraction of waves occurs, resulting in the 'reflection' of signals back down to the earth, whilst at other times signals can be totally absorbed by the ionised gases. During daylight hours solar radiation penetrates the atmosphere far enough to form the lowest layer of ionisation, the 'D' layer roughly 60km above ground.