Plain-language answer: Sunspot number is an index derived from counted sunspot groups and spots, used as a broad indicator of solar activity.
Why operators care
It correlates with long-term ionizing radiation but is not a direct real-time measure of the ionosphere or one path’s usable frequency.
What to compare on air
Use smoothed values for cycle context and current SFI, geomagnetic, X-ray, and ionosonde data for present conditions.
The common mistake
Predicting today’s contact from today’s sunspot number alone.
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.
