Should we delight in a red sky at night?

Kat's been keeping a weather eye on the sky, for this week's mythconception ...

Kat - You've probably heard the old saying "Red sky at night - shepherd's delight; red sky in the morning - shepherd's warning" - although in the US they substitute sailors for shepherds - but is it true?

It's certainly old, as the phrase dates back to at least biblical times. In the book of Matthew, Jesus is quoted as saying "When it is evening, ye say, 'It will be fair weather for the sky is red.' And in the morning, 'It will be foul weather today, for the sky is red and lowering.'" And many of us will probably have gazed at a glowing sunset and murmured the phrase, hoping for a nice day tomorrow, or woken up to a fiery sky and wondered where we put the umbrella.

So what makes the sky turn red in the morning or evening, and does it really reflect the weather to follow? Red skies happen because of the way that molecules of gas and other particles in the atmosphere scatter light. When a beam of white sunlight hits the atmosphere, the shortest wavelengths - blues and purples - get bounced around more easily, pinging off tiny molecules of oxygen and nitrogen in the atmosphere. This is why the sky looks blue during the daytime, because the sun is directly overhead and the blue light is getting scattered all around the sky.

But in the evening and in the morning, when the sun is low in the sky, the light takes a longer path through the atmosphere to get to us. Only the longer wavelength red and orange light get to our eyes on the ground, as the shorter wavelength blue light has already been scattered away. But as any weather-watcher knows, we don't get glorious glowing sunrises and sunsets every single day. And there are a few things that determine whether our shepherds or sailors will be happy or not on any given morning or evening.

To see a red sky at night, or a red sunrise, you need to be able to perceive the light from the sun from the ground. This is, obviously, easier if there aren't any clouds, or if the clouds are high in the sky. And these high clouds (or no clouds) tend to be associated with good weather. But they can also be a forerunner of rain - so you might not notice if it rains during the night after a red sunset, but you'll notice if it tips it down during the day.

Furthermore, high atmospheric pressure - associated with good weather - tends to trap small blue light-scattering particles of dust and pollution. Because weather systems tend to travel from west to east, particularly around the kind of mid-latitudes of the globe like the UK, a red sky as the sun sets in the west means that high pressure - and good weather - is moving in from the west. But if it's already headed over to the east, illuminated by the rising sun, the weather system has moved on and is on its way out, making way for wet and windy low pressure. But this isn't an absolute guarantee, as weather systems can move from south to north, which means that the prediction doesn't work.

Finally, it's often also said that dust and air pollution make sunsets more dramatic, and up to a point this is true, as the small particles scatter short wavelength blue light and leave the red to glow through. But large particles of pollution in the lower atmosphere tend to absorb and scatter all frequencies of light equally, so they actually tend to mute and muddy the colours rather than enhancing them.

So next time you're staring at a romantic sunset, you can turn to your loved one and whisper "Red sky at night, shepherd's delight, but only if the weather is right."