Dr Karl Kruszelnicki

Red sky at night

YOU’VE MOST PROBABLY heard the saying, “Red sky at night, sailors’ delight. Red sky in the morning, sailors’ warning.”

It often holds true for the mid-latitudes, where weather systems usually travel west to east. (Note, however, that while that works in both the northern and southern hemispheres, it doesn’t hold in the tropics near the equator where weather patterns are different.)

The saying relies on the Sun

(when it’s below the horizon) being a ‘probe’ that tells you there are no clouds to your west and that often means fine weather.

First, a bit of background. White light is made up of all the colours of the rainbow, from violet to blue. Raleigh’s Law of Scattering says blue light is scattered much more than red – more than three times more. And when you look at the Sun on the horizon, you’re looking through 38 times more air than when looking straight up.

A lot more air means a lot more scattering. So most of the blue light gets scattered off to the side, where you can’t see it. The remaining light that lands inside your eyeball is overwhelmingly red.

Suppose it’s twilight, with the

Sun well and truly below the horizon. If you’re still seeing a red sky, that means no clouds are blocking the sunlight. (Otherwise, the clouds would block the sunlight and you wouldn’t get that red sky.) So that weather system to your west, where there are no clouds right now, is going to be directly above you tomorrow.

Remember: no cloud often means fine weather.

Now it all makes sense: red sky at night, sailors’ delight. The Sun that has already set is telling you there are no clouds to the west. It’s taken me quite a few years to work this out, and I’m still not fully convinced about the other half: “Red sky in the morning, sailors’ warning.” But when I’m fully convinced, I’ll definitely let you know. DR KARL is a prolific broadcaster, author and Julius Sumner Miller fellow in the School of Physics at the University of Sydney. His latest book, Dr Karl’s Random Road Trip Through Science, is published by HarperCollins. Follow him on Twitter: @DoctorKarl

This sunset over the Port of Melbourne suggests it should have been fine weather the next day.