I’d like to refer to this site that people use for the MUF (Max Usable Frequency):
The MUF depends on the angle of a signal approaching the ionosphere. A shallow angle, like toward the horizon for DX, is used to determine the MUF on the main page. Since most people have a low antenna for 40m/80m/160m most their signal goes upward at a much steeper angle (NVIS cloud warmer), so the FOF2 is more useful since it measures signal propagation at a steeper angle of incidence:
For 80m, when the FOF2 is <= 4Mhz, “the band goes long” because upward NVIS is no longer refracted back to earth, short range signals (including noise) disappear, and only lower angle signals continue to propagate. Distant signals can be more easily heard since local signals disappear which contributes to the band “going long”. There are some similarities between this and a grey line phenomenon, it just doesn’t always line up with a sunset/sunrise.
I monitor this for 80m and 40m a lot, and by watching the FOF2 one can predict when to try local 30m or 20m NVIS as the FOF2 >= these frequencies permits that kind of propagation.