The Answer:

The expression you are referring to has several variations, but this old English sailing proverb seems to be the most common.

Red sky by morning,
sailor take warning.
Red sky at night,
sailor's delight.

This method of forecasting isn't 100% correct all the time, but what weather report is? The simple meteorology lesson behind this proverb is understood best by people in countries where weather fronts tend to move from west to east, which includes the United States.

In order for there to be a red sky in the morning, the sky would need to be clear in the east (where the sun is rising), and the sun's rays would be reflecting off clouds in the west. Those clouds could mean a storm is heading your way, so "take warning."

For there to be a red sky at night, the clouds would have to be moving away in the east as the sun set in the west. In this case the storm has passed, "sailor's delight."

The origin of the red sky proverb can be traced back to the Bible where in the book of Matthew, Jesus says to a group of fishermen, "When it is evening, you say, 'It will be fair weather, for the sky is red' and in the morning, 'It will be stormy today, for the sky is red and threatening."

—The Editors