Red Sky at Night, Shepherd's Delight
"A red sky at sunset predicts fine weather the next day; a red sky at sunrise warns of bad weather coming."
The one that isn't folklore at all. It's Biblical, appearing in the Gospel of Matthew via the 1395 Wyclif translation, and Shakespeare used a version of it in Venus and Adonis (1593), but it also holds up meteorologically. A red sunset happens when sunlight passes through dust trapped in stable, high-pressure air to the west, meaning fine weather is on its way. A red sunrise means that same high-pressure system has already moved through, with lower pressure, often rain, now arriving from the west. It holds specifically in regions like the UK, where weather systems travel west to east.
Myth vs. FactThis is the rare entry on this list that's genuinely, scientifically true, not just an old belief that happens to sound plausible.
Sources: Wyclif Bible, Matthew XVI (1395); Shakespeare, Venus and Adonis (1593); UK Met Office.