As a kid, I remember my mother using this expression countless times:

“Red sky at morning, Sailors take warning. Red sky at night, Sailors delight.”

And as it turns out, this expression been around forever (There are many variables, from many different cultures). It’s even in the Bible.

Matthew 16:3 “And in the morning, It will be foul weather to-day: for the sky is red and lowering.”

Now, I never really figured my mom as a meteorologist, but I did notice that the adage was usually true. But to understand why, we have to understand a little about weather, and the colors in the sky.

Weather typically moves from West to East (blown by westerly trade winds). The sky’s colors are due to sunlight bouncing off water vapor and dust particles in the atmosphere. And a high concentration of them usually appears as a “red” sky.

Keeping in mind that the sun “rises in the east and sets in the west,” a red sky in the morning implies the rising sun in the east is shining on clouds to the west–and conditions are clear to the east. Clouds moving in from the west indicate an approaching storm system.

A red sky at night implies the sun (setting in the west) is shining on clouds to the east and conditions are clear to the west (because the sun can be seen setting). So basically, by the time you see the red sky at sunset, the bad weather has already passed!