The sunset call is Sol's forecast of tonight's colour, in one of four words: little, some, good or exceptional.

Sol issues the call from the forecast at least an hour before sunset and never rewrites it; what the sky then does can be recorded against it in Sky observations, and every call is kept in the open. The four words are deliberately few: a forecast that says more than it knows is worth less than one that says exactly what it does.

It weighs the cloud overhead, the cloud along the light's path to the west, and the air between, from the same forecast twice a day.