Isn long distance HF communications more an infrequent anomaly rather than a common occurrence?
HF propagation in an area varies on the time of day and on solar activity. It’s usually possible to communicate on some HF band to various places in the world 24 hours a day. During the day, the D, E, and F layers form a thicker layer of ionization. This ticker layer absorbs lower frequencies (below 5 Mhz or so), and enables farther propagation of frequencies between 15 and 30 Mhz. At night, the D, E, and F layers combine. This causes the upper frequecy limit (called Maximum Usable Frequency or MUF) to drop, usually to about 14 or 15 Mhz. During this time, frequencies below 5 Mhz will propagate better. This is why you can hear many AM broadcast radio stations at night, and most AM radio stations decrease their power at night to avoid interference.
