A quick tour of the sky

Before any forecast, three basics make the sky legible — the thin layer you live under, what the air is actually made of, and the handful of cloud shapes everything else is built from.

The atmosphere, in layers

Weather — every cloud, every fire sky — happens in the lowest layer, the troposphere: a surprisingly shallow band that reaches only about a dozen kilometres up (a little more over the tropics, less over the poles). Above it the air thins into the stratosphere and beyond. It helps to picture how thin it is: if the Earth were an apple, the whole weather layer would be thinner than the skin. Everything in this library plays out inside that skin.

Slicing the sky by pressure

Forecasters rarely talk in kilometres; they slice the sky by pressure. Air pressure is simply the weight of all the air above you, so it drops as you climb — roughly halving every five to six kilometres. A handful of standard pressure surfaces — 850, 700, 500, 300 hectopascals — act as the sky’s floors, and cloud heights and winds are usually described by which one they sit near.

What the air is made of

Dry air is almost entirely nitrogen and oxygen, with a little argon and a trace of carbon dioxide — and those majority gases barely change from place to place. What decides how the sky looks are the small, variable ingredients: water vapour, which becomes cloud, and aerosols — the fine dust, sea salt, smoke and haze suspended in the air. They are a tiny fraction of the mass, but they do almost all of the visible work: scattering light, seeding droplets, tinting a sunset.

Three kinds of cloud

Almost every cloud is a variation on three shapes. Layered clouds are flat, spread-out sheets, formed when a whole slab of air is lifted gently. Heaped clouds are the puffy, cauliflower towers of warm air rising. Rippled clouds are the wave patterns that show up when a cloud layer rides on moving air. Height matters as much as shape: low clouds sit within a kilometre or two, mid clouds a few, and high clouds are thin ice far overhead — and it’s those high, well-placed clouds that catch the last red light of a fire sky.

Low, middle and high

Clouds are named two ways at once — by shape (above) and by height. The height families are simple: low clouds sit below about 2 km (the flat stratus and puffy cumulus of an ordinary day, and fog lying on the deck); middle clouds ride from roughly 2 to 7 km (the “alto-” clouds — rippled sheets and patches); and high clouds are thin ice from about 5 km up to the top of the weather layer (wispy cirrus and milky veils). It’s the high, well-placed clouds that most often catch a fire sky, and the low ones that flood a valley for a sea of clouds.

Where clouds come from

A cloud is just air that has cooled to the point where the water vapour in it can no longer stay invisible. Cool a parcel of humid air — by lifting it over terrain, sliding it along a front, or simply by letting it radiate heat away on a still night — and at its dew point the vapour condenses onto aerosols into countless tiny droplets. That is a cloud. The same nightly cooling that makes valley fog is what fills the low ground for a sea of clouds; the same lifting that builds a storm can leave behind the high screen a sunset needs.

With these three — a thin weather layer, an air whose trace ingredients do the visible work, and a small family of clouds — the rest of the sky’s rare shows stop looking like luck.