Quote:
I think it promises a clear day tomorrow

Isn't that just what it means? A segment of promise ? \:\)

Often the sunset sky predicts the weather for the next day.
Yesterday at sunset there was a deep red sun sort of dropping from dark grey clouds. That means rain and sunshine for now. A red dawn promises rain.
It's caught in a popular saying:
" morning-red, water in the ditch; evening-red, nice weather aboard." (can't make it rhyme in English)