How to read the propagation indices: SFI, SSN, K and A
· 8 min read
Every conversation about HF conditions comes down to four numbers: SFI, SSN, the K index and the A index. They get passed on the air, printed in forecasts and written in logs — but not everyone can read them.
In short: the first two say how much the Sun is giving out (and therefore how dense the ionosphere is), and the last two say how quiet the Earth's magnetic field is (and therefore whether a storm is choking that ionosphere).
Below is a full account of each index: what it means physically, how to read it, what it affects and when it is worth getting on the air. Beside each section there is a live gauge that pulls the current value from NOAA and shows where we stand.