Red sky at night
Red sky at night often does mean a fair day ahead where weather travels west to east, because the setting sun lights the clouds only if the western horizon is clear. The saying is about looking toward where tomorrow comes from.
What it may mean
The red-sky saying has lasted because it is often right. In much of North America and Europe, weather systems travel from west to east. A sunset sky that glows red tells you the sun is shining through clear, dry air to the west and lighting the clouds overhead from below, and that clear air is what will drift in by morning. The saying works by looking in the right direction: toward where tomorrow's weather is coming from. People rarely do the same with their own weeks. We study today's sky, the mood of the moment, instead of glancing west at what is already on its way: the calendar, the deadlines, a visit, a rest on the way. A red sky is a good reason to look ahead in the direction your days actually come from. The rhyme works best in the middle latitudes.
Depending on how you saw it
When the dawn is red instead
The same rhyme warns about a red sunrise. The low morning sun lights clouds that have already moved in from the west, so wet weather may be close. Which early sign in your morning tells you how the day is likely to go?
A sunset after a long gray day
When clouds break at the very end of a dull day and the west lights up, the effect is dramatic because you had stopped expecting it. A good ending can change how a whole day is remembered. How could you give your own days a better last hour?
A red sky over the sea
At sea, reading the evening sky was once practical knowledge, since a wrong guess about tomorrow's wind could be dangerous. If your plans for tomorrow depended on it, what would you check each evening?
Why it happens in nature
At sunset, sunlight crosses a long stretch of atmosphere, which scatters blue light away and leaves reds and oranges. When the air to the west is clear and clouds lie overhead, they catch that red light from below. Because mid-latitude weather mostly moves from west to east, a clear red sunset often comes before fair weather.
To reflect on
What is already heading your way this week, if you look in the direction your days come from?