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 (1). 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!

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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.