Why the gray line works
On the daylight side the D layer absorbs low-frequency signals. On the night side the D layer is gone, but the F layer has lost some of its ionisation. Along the terminator the D layer is fading (or not yet formed) while the F layer is still ionised, so absorption is low and the F layer still reflects. Signals can follow the terminator for very long distances, and you can work stations that are also in twilight at the other end of the path.
How to use it
- Find sunrise and sunset times for your location and for the target: use the sunrise and sunset calculator by grid locator.
- Operate on 160, 80, 40 and 30 metres around your own sunrise and sunset, when the path to a station in twilight lies along the terminator.
- Windows are short, often 30 to 60 minutes, and they shift a few minutes every day with the seasons.
- At the equinoxes the terminator runs north to south, which favours east-west paths. In the northern summer and winter it tilts, which favours some north-south paths.
See it live
The live gray line map draws the terminator and the subsolar point in real time. Stations close to the dashed line are in the gray zone right now.
Keep reading
What Is HF Propagation?HF propagation explained for radio amateurs: how the ionosphere bends 3-30 MHz signals, why bands open and close, and what MUF and skip distance mean.HF Bands by Day and NightWhich HF bands work during the day and which at night, from 160 m to 10 m: typical behaviour, best times and what changes with the solar cycle.Solar Flares and Radio BlackoutsHow solar flare classes A, B, C, M and X work, what the NOAA R1-R5 radio blackout scale means, and how flares knock out HF on the sunlit side of Earth.
