See Propagation on the East Greenbush Amateur Radio Association web site (W2EGB.org).
Propagation is the ability of radio waves to travel beyond line of sight. This may manifist itself by the waves bending as they encounter different layers of the ionoshpere (most likely on the lower frequencies) or by other factors, such as meteor scatter or troposhperic ducting (more common on higher frequencies). With respect to ionospheric propagation, the longer the wavelength, the greater the bending for a given degree of ionization and the greater the absorption as the wave moves through an ionized layer; absorption also increases with intensity of ionization and density of the atmosphere.
It is important to note that low frequencies bend but are attenuated while higher frequencies are less attenuated but are less prone to propagation; therefore, use of the highest frequency that will propagate (i.e., bend) will provide the best signal due to its lessened absorption. This principle will maximize propagation while minimizing signal loss due to absorption. The highest frequency that will propagate while not being absorbed is known as the Maximum Usable Frequency (MUF).
There are three principal layers of the ionosphere: the D-layer is the lowest, almost totally absorbing the low bands during the day when its ionization is proportional to the height of the sun - - only higher angles of radiation can pass through this layer to the higher layers; the E-layer, at about 70 miles, only holds ionization when in continuous sunlight - - being greatest at local noon, giving maximum single-hop distances of about 1250 miles - - and disipates nearly totally with sunset; the F-layer, at approximately 175 miles at night, slowly decreases in ionization after sundown, reaching its minimum just before sunrise - - when it has split into two layers at about 140 and 200 miles - - it gives maximum single-hops of about 2500 miles. There follows a description of how these elements manifest themselves during the course of a typical day.
The D-layer, the lowest level of the ionospher, is heavilly ionized during daylight hours; so too is the E-layer. On the low bands, such as the a.m. broadcast and the 160 and 80 meter amateur bands, signals beyond the local area (the ground wave) are totally (more or less) attenuated as they strike the D-layer: there is local reception only. At sunset, the D- and E-layers dissipate (pretty-much immediately - - sometimes even before sunset); The F-layer holds and propagates on these frequencies now that the waves can reach it, freed of the absorption problems of the lower layers. Although the F-layer dissipates overnight, it will hold on these low frequencies all night long, continuing propagation until the E- and D-layers reform after sunrise.
The higher HF bands, the 40, 20, 15, and 10 meter amateur bands, for example, will not be absorbed by the D- and E-layers during the daylight hours even though these layers are heavilly ionized: the F-layer (and the E-layer) propagate. At sunset, ionization decreases across all layers: the highest frequencies drop first and cease propagation. The F-layer continues dissipating overnight, with propagation degrading into the lower bands. The highest frequency that will propagate continues to drop until sunrise and the reformation of ionization.
Propagation virtually ceases above the 6 meter (50 MHz) amateur band. There will be occasional sporadic E, scattered dense patches in the E-layer: this occurs in temperate regions in the late spring, early summer, and early winter in mid morning and early evening. There will occassionally be troposhperic ducting, which occurs when adjoining air masses in the tophosphere form what is, in effect, a wave guide, and "duct" VHF or UHF signals for what may be many miles. There may also be meteor scatter, propagation in the upper atmosphere that is the result of meteor particles reflecting signals, which is - - predictably - - rather rare.
Propagation on the frequencies at and below the a.m. broadcast band are mostly of the nature of ground waves, which, on those frequencies, will follow the curve of the Earth. These frequencies comprise some maritime stations, aeornautical beacons, and long-range navigational and military communications facilities, not generally of use to the amateur.
An interesting element to this pattern of propagation is called "gray line." The gray line, also sometimes referred to as the terminator, is the line of sunset/sunrise around the world. Along the gray line, extremely long signal paths are possible. One major reason for this is that the D-layer, which - - as noted above - - absorbs HF signals, disappears rapidly on the sunset side of the gray line, and it has not yet built upon the sunrise side; meanwhile, the F-layer is heavilly propagating. The gray line is not a simple north/south line running around the globe from pole to pole across the equator; rather, it presents numerous east/west possibilities; for example, in mid-summer, it is possible to have a gray-line path from North America to Europe, specifically, from evening in central North America to morning in eastern Europe. This is in fact logical, as North America is about six hours ahead of Europe, so late dusk in Chicago (about 9:00 p.m.) would be early morning twilight in Rome (4:00 a.m.).
