Sunlight looks white, but it carries every colour of the visible spectrum at once. As it streams through the atmosphere it meets molecules of nitrogen and oxygen, each far smaller than a wavelength of light. These molecules scatter the light in all directions, and they do not scatter it evenly. The shorter the wavelength, the more violently it bounces around: violet and blue light scatter roughly nine times more strongly than red. That scattered blue reaches your eyes from every part of the sky at once, which is why the whole dome above you glows rather than just the patch around the sun. This is Rayleigh scattering, named for the British physicist Lord Rayleigh, who worked out the mathematics behind it in the 1870s.

A fair question, since violet scatters even more strongly than blue. Three things conspire against it. The sun emits less violet light than blue to begin with. The upper atmosphere absorbs a portion of what remains. And the human eye is simply less sensitive to violet, because our cone cells respond most enthusiastically to the blue and green regions of the spectrum. Mix what physics delivers with what biology can detect and you land on that familiar mid blue. The colour of the sky is a negotiation between sunlight, air, and the eye reading them both.

Winter's first trick: the air dries out

Here is where the seasons come in. Cold air holds far less water vapour than warm air, and the amount roughly halves with every ten-degree drop in temperature. In summer, all that moisture feeds haze: tiny droplets and swollen aerosol particles hanging in the lower atmosphere. These particles are hundreds of times larger than air molecules, and large particles scatter all wavelengths almost equally, a process called Mie scattering. Scatter every colour equally and you get white. That white light dilutes the blue, which is why a humid summer sky so often looks pale, milky, and full of glare. Winter strips the moisture out. With less haze in the way, the pure molecular blue of Rayleigh scattering comes through undiluted, and the whole sky deepens.

Winter's second trick: the low sun

The sun rides lower across a winter sky, and that changes the light twice over. Sunlight arriving at a shallow angle travels a longer path through the atmosphere, so more of its blue is scattered away before it reaches you. The direct light turns warm and golden, which is why photographers adore winter light, while the sky overhead keeps its saturated blue. The contrast between golden light below and deep blue above makes the colour feel richer still. There is a bonus for the curious: skylight is most strongly polarised at ninety degrees from the sun, and with the sun sitting low, that deeply polarised band arcs conveniently overhead. A pair of polarised sunglasses will reveal it. Tilt your head and watch the sky darken and brighten.

The same physics runs in both hemispheres, six months out of phase. A late-summer afternoon in London or New York sits under the bright, hazy glare of a high sun and humid air, the blue washed towards white. That same afternoon in Sydney, the air is cold, dry, and clean, and the sky is near its deepest blue of the year. Neither sky is broken. They are the same scattering physics dialled to different settings, and within a few months each hemisphere trades its sky for the other's. If your winter has just ended, that blue is already booked in for next year. If yours is approaching, the best skies of the year are on their way.

Nature makes colour in four ways. Dispersion splits light apart in prisms and rainbows. Interference folds light against itself in bubbles and oil slicks. Scattering builds the blue sky. And selective absorption subtracts colours out of white light. That fourth trick is the one a CMY Cube performs. Hold one up to a bright winter window and look through the cyan face: cyan absorbs red and passes blue and green, arriving at a colour strikingly close to the one the sky reaches by scattering alone. Two completely different roads, one kindred blue. Slide the magenta face behind it and watch the colour shift again, the same subtraction at work in every printed photograph. The sky scatters. The cube absorbs. Both are ways of coaxing colour out of plain white light.

The next clear, cold morning, take ten seconds before the day begins and look up. That blue is sunlight being taken apart by the air itself, molecule by molecule, on a scale no instrument could ever stage. Winter has a reputation for grey, but its clearest days answer with the purest blue of the year, the sky showing you exactly what it is made of.