I have been reading about the virtual plethora of variables that have an effect on HF propagation. Some make sense while others seem less. The three consistent (within various sources) numbers that give us usable information are the Flux, A Index and K Index.
The following is a summary of what I have been able to find. If you see any errors, PLEASE let me know and I'll correct this page.
If you choose to share the information contained herein, please do it via link rather than full copy. I do not own any of the information here but I spent many hours compiling this information into a form that is easier (at least to me) to understand than what I found elsewhere. I say this because I have found MANY exact (full) copies of web pages from my site, posted on other sites with no indication of where they were copied from.
The intensity of RF at 2800 Mhz. is measured regularly by NOAA and other agencies throughout the world. This number is called the solar flux and provides a direct reference to overall solar activity. This number can vary from values below 50 to values greater than 300. Values in excess of 200 occur during the peak of the 11-year solar cycle. The solar flux is closely related to the amount of ionization taking place in the F2 layer. High solar flux values generally indicate good ionization for long distance communications.
There is a direct correlation between solar flux and the number of sunspots. An increases in solar flux indicates and increase in sunspot activity. This activity is cyclic in nature and tends to vary with the rotation of the sun. This cycle is about one per twenty eight days (27.5 but who can count that closely with everything else changing).
The A index represents the severity of magnetic fluctuations measured at various observatories. This index will vary from observatory to observatory because they are local in nature. We who live on the front range of Colorado are very fortunate in that these measurements are taken in Boulder and are reported via W-W-V radio.
The A index is a calculated from the eight day averaged K index (see below). During magnetic storms the A index may reach as high as 100, while during severe storms the A index may exceed 200. Or, more simply, the higher the number the worse the propagation. A falling A Index trend usually indicates good long distance communication.
The K index compares the H and D magnetometer traces (Horizontal and Declination) reference a "quiet" day (no activity). These results are fed into an almost logarithmic calculator to yield a number between 0 (no activity) and 9 (extreme activity).
Each UT day is divided into 8 three-hour intervals, starting at 0000 UT. In each 3-hour period, the maximum deviation from the quiet day curve is measured for both traces. These results are reported to the public.
The K index is useful in determining the state of the geomagnetic field, the quality of radio signal propagation and the condition of the ionosphere.
Once all three of those numbers seem to make sense, then you need to look at what is happening within the D region of the ionosphere. Since the D region provides us with a lot of the long distance capability we so much like, we should also remember that it can also absorb a lot of the signals we generate.
In the links section above you will note that there is a link for "D region absorption". It is well worth your time to check that map before attempting any regularly scheduled nets. Many days there will be no absorption, while on others there may be as much as 35 db. Think about that, at 30 db absorption a 100 watt signal would sound like a ONE TENTH watt signal. If you are attempting voice communication, that is not very much to work with.
The last consideration is if you wish the HF version of local communication (25 to 250 mile distance). For that the easiest way is to use a Near Vertical Incident Sky-wave (NVIS) antenna. Please see the NVIS Antenna page for more details.
The Australian Government posts a color coded map that give a good indication of the highest frequency that will support NVIS operation. You can find that information at my NVIS Propagation Maps page.
More than that I don't know.