Unlike local radio broadcasting on the VHF (FM) and medium wave bands, short-wave reception relies upon the refraction of signals from layers of gas hundreds of kilometers above the Earth’s surface (known as the ionosphere)! These layers of gas are ‘excited’ when the Sun shines on them, changing their characteristics: some of them become more reflective to radio signals when subject to solar radiation whereas some start to absorb radio signals. This is why some short-wave frequencies are good for night-time reception and others for the day-time. Sunset and sunrise present some other interesting opportunities as both the night-time and day-time layers of gas begin to swap over. This complex interplay of radio signals and layers of the Earth’s atmosphere is usually termed ‘propagation’.
http://www.short-wave.info/index.php?feature=propagation
Interesting.
Thank you:)