DP HC: The Green Flash
At sunset, the sky sometimes lights up green. This is often called the Green Flash, Green Ray, the Mock Mirage, or M-Mir.
Scientifically speaking this flash is "as the sun sinks its rays start to pass through the temperature inversion. The action of the inversion is most easily understood by concentrating on the interface between the warmer layer and the cool more dense air beneath. Rays passing downwards through it are refracted so that they curve slightly towards the earth. The upper parallel sun ray glances through the inversion and is most strongly curved. To the eye, the ray appears to be coming from a point higher in the sky and it contributes to an upper (and inverted) image of the sun. The lower parallel ray is less deflected and contributes to a lower solar image. In a direction tangential to the base of the inversion layer small vertical differences in ray directions are highly magnified. The normally insignificant green rim of the sun is vertically stretched to produce a green flash."
But mythologically, it's said that this happens when a soul returns to Earth from the afterlife.
The truth is that the Green Flash takes place whenever the mnemonic imprint of a spirit from Earth journeying through the Infinite Realms erodes completely. At which point, they disappear from the IR & reappear back on Earth to be reincarnated as someone new.
This always happens at sundown & is usually due to a large number of souls returning at once. The reason why it's green is due to the burst of ectoplasmic energy that takes place when the unbound souls shift from one universe to another.
No one in the Living Realms knows why it has to happen like this, but when a soul is ready to reincarnate, it disappears, but doesn't immediately reappear in it's home universe. You see, it takes a lot of energy to breach the barrier between universes, so many reincarnating souls must gather, it's suspected in some form of pocket universe, & their soul magic, blue magic specifically which is associated with spacial/gravity magics & teleportation, builds up more & more until, with an ectoplasmic boost, there's enough to carry them all back.
No one knows, specifically, why reincarnating souls only return at sundown.
The truth of the matter is that rebirth deities such as Zagreus gather these souls & use soul magic to take them back to the Living Realms. This sudden appearance creates a great flash of green light in the sky due to the ectoplasm released in the area.
If you want to see my full Ghost Zone Masterlist, go here. You can find out more about ghosts & the reincarnation process from "Explaining the Ghost Zone," watch out, it's a long post.