Have you ever wondered why the sky is blue during the day, and why it can shift to reds and oranges during sunset and sunrise?

This ubiquitous change in colour has inspired old sayings, like “Red sky at night, shepherd’s delight. Red sky in the morning, sailor’s warning.” In the northern hemisphere, or at the very least in Britain, red sky at night often signals clear skies and calm weather, while a red sky in the morning can indicate that the good weather has already passed by.

So why is this?

The light from our sun is white, meaning it contains a wide range of wavelengths, and therefore, colors. Interestingly, sunlight peaks in the blue-green part of the spectrum, which is why plants are green and why humans are particularly good at distinguishing shades of green more than other colors.

But if sunlight is white, why does the sky look blue? If you look at photos of Earth taken from space, you’ll notice that only the areas where sunlight passes through the atmosphere appear blue. This tells us it’s not the sunlight itself that’s blue, but rather how the light interacts with the air molecules. (Please don’t look directly at the sun!)

If air itself were blue, then the sky would almost always appear uniformly blue, much like looking through blue-tinted glass—but that’s not what we see. This effect was observed in the 1800s by a man named John William Strutt, also known as Lord Rayleigh. He was a Nobel Prize-winning mathematician, and discovered that the blue color of the sky is due to sunlight being scattered by air molecules, a process now known as Rayleigh Scattering.

He explained that light waves can’t pass through air molecules without being redirected, and shorter wavelengths, like blue, are scattered more than longer wavelengths, like red. This scattering is what gives the sky its characteristic blue color.

To put it simply, the light you see directly from the sun is mostly white – minus a small amount of blue that has been scattered away, meaning that the sun actually appears slightly less blue than it would if viewed from space. On the other hand, the indirect light you see, which has been scattered in different directions before reaching you, is more blue.

So why do sunsets and sunrises appear more red than blue? When you look at a sunset, you’re gazing in the direction of the sun, meaning – as said before – there’s less blue light in that direction. As the sun is ‘low’ in the sky, its light must pass through more of the atmosphere, scattering even more blue light away and leaving the longer red wavelengths to reach your eyes.

This effect becomes stronger the more air the sunlight has to pass through. In warmer, denser air, more blue light is scattered, resulting in deeper red sunsets. That’s why sunsets in warmer climates or during the summer often appear more spectacular than those in cooler regions.

This is where the phrase “Red sky at night, shepherd’s delight. Red sky in the morning, sailor’s warning.” comes from. In Britain and other places with predominantly west to east weather systems, seeing a red sky at night often means that warm weather is on it’s way, as the air that the sunlight from the west passes through is more dense and could contain dust or other small particles. Conversely, a red sky in the morning signifies that warmer air is to the east, where it may have already passed you by.

There’s a lot to be learned by asking questions about some of the things we take for granted every day. Something as simple as the colour of the sky has captured the imagination of people for all of history, and we’ve only been able to explain it for less than 300 years.