By Randall Munroe, as printed in the New York Times Aug. 13, 2019
According to popular lore, you can predict the weather based on sky color. The saying typically goes, “Red sky at night, sailor’s delight. Red sky at morning, sailors take warning.”
This saying has been around in various forms for a long time — there’s a version of it in the Bible. The reason it’s lasted so long is that it actually works, at least in certain parts of the world. The red sky method doesn’t have as much to do with the red clouds themselves as you might think — instead, it’s a way of using the sun to do an X-ray of the atmosphere over the horizon, and then using clouds above you as a screen on which to project the results!
In the temperate zones, weather systems generally move from west to east. They don’t move too fast — in general, weather moves across the Earth at driving speed or slower — so a storm system a thousand miles to your west won’t reach you for a day or so. Because of the curve of the Earth and the haze of the atmosphere, you can’t see the clouds to your west; if you could, weather forecasting would be a lot easier.
The “red sky” trick gets around this by using the sun. Red wavelengths pass through air more easily than blue ones. When the sun is setting in the west, its light passes through hundreds of miles of atmosphere — becoming extremely red in the process — before hitting the clouds above you. Shorter blue wavelengths bounce off the air and go off in other directions. This is why the sky is blue — it reflects blue light. White clouds reflect all colors, so when red light shines on them, they look red, too.
If there are storm clouds to your west, the red sunlight is stopped before it can get to you, and the sunset doesn’t look particularly red. On the other hand, if there’s clear air for hundreds of miles to your west, the sunlight passes all the way through to reach the sky above you, turning it red. If there are any clouds overhead, the red light illuminates them, creating a spectacular sunset.
When weather moves west to east, a red sky at night means that there are clouds overhead, but clear skies to the west — which tells you that the weather will likely be clearing up.
A red sky in the morning, on the other hand, means that there’s clear air to the east . . . but clouds overhead. That means the clear zone is moving away, and clouds are moving in.
This saying doesn’t work in the tropics, where prevailing winds tend to move east to west and are generally more unpredictable. On the other hand, weather in the tropics is much more stable — excepting the occasional unpredictable cyclone — so there’s less need for this kind of rule of thumb.