The image below is an animated GIF of G4ILO's VOAprop program, an HF propagation modeling tool. VOAprop downloads publicly-available solar activity data from the Internet, takes into account the time of day and time of year, massages it with a complex set of formulas developed by the U.S. government ITS (Institute of Telecommunications Sciences) Propagation Laboratory, and finally plots the results on a world map.
This sequence of 24 screen shots, taken 1 hour apart, represents one 24-hour period on September 4, 2009, and shows the expected coverage, hour-by-hour, on 30 meters (10.1 MHz) from my former home location in southern California. The "High-Medium-Low" slider is a correction factor to compensate for antenna performance. I have set it some-what less than average to simulate transmissions from a mediocre mobile antenna being received on a simple dipole at the fixed station.