Afterglow is the colour that lingers on the western sky after sunset, lit from below the horizon by a sun you can no longer see.

After sunset the ground is in shadow but clouds a few kilometres up are still in direct sun for another quarter of an hour, lit from below through a long, reddening path of air. High, thin cloud gives the best of it; a clear west lets the light through, and an overcast one kills it — which is exactly what the sunset call weighs.

It is at its best a quarter of an hour after sunset, when high cloud catches light that the ground has lost.