The phrase “red sky at night, sailors’ delight; red sky at morning, sailors take warning” is a common rule of thumb for keeping tabs on the weather. Even the Bible gives it a mention: Matthew 16:2-3 says, “When it is evening, ye say, fair weather: for the heaven is red. And in the morning, foul weather today for the heaven is red and lowering.” It may surprise you to learn that this saying has scientific validity.
In the mid latitudes, weather systems tend to travel from east to west. Red skies occur when sunlight is scattered through suspended particles in the atmosphere, which are at their greatest in the sorts of high-pressure systems that bring the best weather. So if a bad weather system was leaving as the sun sets in the west, the sun would illuminate the departing clouds around the western horizon and create a “red sky at night.” Likewise, if a bad weather system was moving in as the sun rises in the east, the sun would illuminate the approaching mid- and high-level clouds to create a “red sky at morning.”