HF propagation (3 to 30 MHz) bounces signals between the ground and the ionosphere, the band of charged particles 60 to 600 km up that gets ionized by the sun. The same QSO at 3 PM and 3 AM can be trivial or impossible depending on how the ionosphere is acting, which is why ham operators watch a handful of indices to predict band conditions.
Measured at 10.7 cm (2,800 MHz) in solar flux units (sfu). Higher SFI means more solar UV reaching the F-region of the ionosphere, which means higher MUF (Maximum Usable Frequency), which means the higher HF bands (17m, 15m, 12m, 10m) start opening up.
Rule of thumb:
long-haul; high bands dead.
night; 17m and 15m daylight.
worldwide DX.
Measures geomagnetic disturbance. K is a 0-9 logarithmic scale updated every 3 hours. A is the daily quasi-linear average. Higher values mean Earth's magnetic field is being shaken by solar wind, which absorbs HF energy and degrades propagation, especially through the auroral oval.
Rule of thumb for K:
long-haul.
hours, especially at higher latitudes.
The highest frequency that bounces off the ionosphere instead of punching through it. Above the MUF for a given path, your signal escapes into space. Below it, the bounce works. MUF varies by:
angle is shallower)
pre-dawn)
The /propagation page shows current band-by-band conditions and the live MUF estimate so you do not have to do this math in your head.
A typical HF operator at the start of a session glances at:
ionosphere on the right side of dawn / dusk?
The /greyline tool layers terminator math (where dawn / dusk is on the planet right now) on top of the indices so you can see at a glance where the openings actually are.
Astrophysical Observatory in Penticton, British Columbia, and republished by NOAA SWPC.
observatories worldwide.
band-by-band "Good / Fair / Poor" predictions that hams actually use.
ham-readable summary feed used on /propagation.
fuller treatment of the physics.