Day and night
In daylight the D layer absorbs low frequencies, so 160, 80 and 40 m are weak over long distances, while the F layer is dense enough to carry 20 m and higher bands. At night the D layer disappears and the F layer weakens: the low bands open and the high ones close. As a first approximation: 160 m, 80 m and 40 m work best at night and around sunrise and sunset; 30 m is usable by day and by night; 20 m is the all-round DX band by day and often into the evening; 17, 15, 12 and 10 m depend on the solar cycle and are best around the middle of the day.
The sun and the seasons
The solar flux raises the ionisation: with a high flux the MUF climbs and 10 m can open to the whole world; near the minimum of the cycle it is often closed. Season and latitude change the picture, and a geomagnetic storm (high Kp) can depress the MUF and the absorption at high latitudes. Check the current values and your own path on the coverage map, which shows the matrix of bands by hour and the MUF.
Skip zone and hops
Because the signal returns from the ionosphere, there is a ring around the transmitter where nothing is heard, the skip zone, and the distance grows with the frequency. A long path uses more than one hop, each adding ground losses and absorption. See HF skywave versus VHF line of sight for the physical difference with higher bands.
