Why Do Those Wispy Clouds Never Rain? They Are Made of Ice, Miles Up

These clouds form at heights above roughly six kilometres, where the temperature is well below minus thirty and no liquid water can persist, so they consist entirely of ice crystals rather than the droplets that make up lower cloud. Those crystals are large enough to fall slowly, and they do, but they fall into air that is dry and evaporate before reaching the ground, which produces the characteristic trailing streaks. The wind at that height is generally much faster than at the surface and varies with altitude, which draws the falling streaks into hooks and fibres.

Watching them is a genuinely useful forecasting habit:

• •Thickening and lowering streaks frequently precede a warm front
• •That arrangement suggests rain within a day or so
• •Streaks with strongly hooked ends indicate strong wind shear aloft
• •A sky that stays clear behind them suggests nothing is coming
• •They produce halos, which are a separate and reliable sign
• •They are the only clouds normally visible from a passing aircraft above

The optical effects associated with them are a direct consequence of the crystal shape. Ice crystallises in a six-sided form, and a hexagonal prism bends light passing through two of its faces by a minimum angle of about twenty two degrees, so a sky full of randomly oriented crystals produces a ring of that radius around the sun or moon. Crystals that fall with a preferred orientation produce brighter patches at the sides of that ring instead. A halo therefore indicates high ice cloud specifically, which is why the folk observation connecting a ring around the moon with approaching rain has a real mechanism behind it.

Aircraft produce these clouds directly and the mechanism is specific. Engine exhaust contains water vapour and particles, and injecting both into air that is already cold and close to saturation causes ice to form on those particles almost immediately, producing a line of ice cloud behind the aircraft. Whether it persists depends entirely on the humidity at that height, so a trail that vanishes within seconds and one that spreads across the sky for hours are the same process meeting different air. Persistent spreading trails are indistinguishable from natural cloud within an hour, and estimating how much of the high cloud over busy air corridors is artificial is an active research problem.

Cloud affects the climate in two opposing ways and which dominates depends on the cloud. All cloud reflects incoming sunlight back to space, which cools, and all cloud absorbs heat radiating up from the surface and re-emits some downward, which warms. Low thick cloud is bright and reflects strongly while its top is warm, so the cooling dominates. High thin cloud reflects little because it is thin, while its top is extremely cold and therefore radiates very little energy away to space, so the warming dominates. That is why aircraft contrails, which are artificial versions, are a recognised contribution to warming quite separate from the fuel burnt.

Above six kilometres no liquid water persists, so these clouds are ice crystals that fall, evaporate before landing and get drawn into streaks by wind that changes with height. Hexagonal crystals bend light by about twenty two degrees, which is why they produce halos. Being thin they reflect little sunlight, and being very cold they radiate little away, so they warm rather than cool.