The “grey line” is a term you hear often in ham radio, especially from DXers. It refers to the zone where the Earth is transitioning from night to day — the twilight band circling the globe during sunrise and sunset.
Why is it special? Because the ionosphere behaves differently along that boundary. The D layer (which normally absorbs lower HF) fades away rapidly while the F layer (which reflects) remains strong for a while. That makes it an ideal time for long-distance contacts, especially on bands like 40m or 20m.
If you’ve ever had a run of unexpected DX just after sunrise or just before sunset, that’s probably the grey line at work.
You can take advantage of it by timing your operating for twilight at your QTH or your target station. Tools that overlay the day/night line on a map help you visualize it — so you can aim for regions just entering or exiting daylight.
Like most things in radio, it’s not a guarantee. But it’s one of those rare moments where propagation favors the patient — and the observant.