The field meaning: Gray-line operation exploits short-lived HF path changes near dawn and dusk, when D-region absorption falls rapidly while higher ionization may still support refraction.
What is really happening
The best band depends on both endpoints, path orientation, season, solar state, and whether the useful segment follows darkness, daylight, or the terminator.
Do not confuse it with this
A shaded terminator map is geometry, not a propagation measurement; it cannot show actual absorption, MUF, noise, or antenna pattern.
Use it deliberately
Prepare several bands before the window, compare sunrise/sunset at both ends, listen to beacons and known stations, switch quickly, and log which path and frequency improve.
Decision boundary
Operating shorthand and propagation models are tools for making a better decision, not proof of a path, contact, or safety condition. Confirm with current rules, measured signals, weather, and the complete station.
