Plain-language answer: An HF radio blackout is substantial sunlit-side absorption caused by enhanced D-region ionization, commonly during strong solar X-ray flares.
Why operators care
Lower HF frequencies are usually affected most, onset can be rapid, and the dark side may remain comparatively normal.
What to compare on air
Check X-ray flux and solar illumination, try higher authorized HF frequencies, and wait for absorption to decay.
The common mistake
Retuning or dismantling the antenna during a propagation-wide blackout.
Forecast boundary
Space-weather indices and propagation models describe probabilities and regional conditions. A usable contact still depends on the exact path, time, frequency, antenna pattern, noise, power, and current observations.
