Helen Czerski

Helen stares up at the clouds.

Once in a while, it’s worth looking up. Our busy lives mean that we’re tuned into streets, buildings, cars and kitchens – a rich world of detail at eye level or below. But we often neglect the vast amount of free space above our heads. Our atmosphere is mostly just air and water, but the visual variety of the sky is astonishing, especially when it comes to the clouds. How does such colour and complexity come from these simple ingredients? The answer is that clouds give us a sneak peek at the optical traffic above our heads – the light that wouldn’t normally make it to our eyes.

There is so little to a cloud. They’re made of water droplets and ice crystals that are typically a hundredth of a millimetre in diameter, with a few hundred droplets in each cubic centimetre. One cubic metre of puffy ‘normal’ cloud contains around a quarter of a gram of liquid water. But those tiny droplets of water get in the way of light. Light entering a cloud plummets into an optical pinball machine, scattered again and again by the droplets until perhaps it finds its way out. This process is such an obstacle that you can see through heavy rain far better than fog, even though there’s far more liquid water in the rain, because the larger raindrops have bigger gaps between them.

If white light goes into the cloud, white light will come out. We only see the cloud because it redirects light from the Sun. But as clouds get thicker, it’s less likely that light will make its way all the way through the pack and out on the underside, which is why clouds are light at the edges and darker grey underneath.

Then there are those days when the skies darken and we know that rain is due. Raincloud droplets are larger, and potential raindrops are bigger still. Each light ray has to pass through much more water, partly because the cloud itself is thicker. A growing thundercloud might still only have 1.5 grams of water per cubic metre, but it’s kilometres tall and light doesn’t stand a chance of getting through. Dramatic dark is a fairly reliable indicator that rain is likely.

But it’s sunrises and sunsets that really allow the clouds to bask in reflected glory, because the air itself isn’t perfectly transparent. Air molecules and tiny particulates will scatter the violet and blue end of the rainbow, leaving a direct beam made up of reds and yellows. But this filtering is only noticeable at sunrise and sunset, after light has passed through hundreds of kilometres of air as it travels across the sky instead of straight down to Earth. And so at sunset, giant sheets of filtered light from the Sun whoosh over our heads, yellow higher up, with pinks and red lower down, on their way out into the cosmos. This atmospheric party of colour passes us by unless clouds are parked in its way, reflecting the glow. The colours are always there but the clouds curate a shifting exhibition of the sky, letting us in on the secret.

Early summer evenings are the perfect time for appreciating all this. So next time you need a rest from the bustle of life on the ground, I recommend spending a bit of time staring at the sky, where air, water and light are always putting on a show.