The Types of Ice

Clear ice: the slow freeze

Clear ice (also met as glaze) is a transparent, glassy, dense sheet of solid ice, formed when large supercooled droplets freeze slowly enough to spread over the surface before solidifying. Its conditions follow straight from the previous lesson's supply curve and droplet pairing: large droplets (cumuliform cloud, freezing rain or freezing drizzle) at temperatures just below 0 °C, roughly 0 to −10 °C, the warm top of the supercooled range where the drops are biggest and wettest. The result is the accretion the next lesson will rank most dangerous: smooth or lumpy, heavy (no trapped air means full ice density), hard, and tenacious: it grips the surface, spreads aft beyond the protected leading edges, resists removal, and when it finally lets go it leaves in large, unbalancing chunks.

Why the drop freezes slowly

The slowness is the thermodynamics module's latent heat, working against you. When a supercooled drop strikes the wing and starts to freeze, freezing releases latent heat, and that heat warms the rest of the drop back toward 0 °C, so only a fraction freezes on impact. The arithmetic makes it vivid: freezing a gram of water releases enough heat to warm the rest by many degrees, so for each degree of supercooling, only about one-eightieth of the drop can freeze instantly. A big drop at −5 °C therefore hits as mostly liquid water, flows aft over the surface, and freezes gradually into a continuous, air-free, transparent sheet — which is the whole recipe for clear ice, and why it forms best where the air is only slightly below zero.

Rime ice: the instant freeze

Rime ice is the opposite story: a white, opaque, granular deposit, formed when small supercooled droplets freeze instantly and completely on impact, trapping air between the frozen droplets. Its conditions are the cold, lean end of the supply: small droplets (stratiform cloud, low water content) at colder temperatures, roughly −15 °C and below, where the drops are tiny and freeze before they can spread. The trapped air dictates every aspect: white and opaque, rough and granular, light, and brittle — it builds forward into the airflow on the leading edges rather than flowing aft, crumbles and breaks away comparatively easily, and troubles the aircraft more by roughening the wing than by weighing it down.

Mixed ice

Clouds rarely read the textbook, and when a cloud offers both large and small droplets (or supercooled drops together with snowflakes and ice crystals) the deposit is mixed ice: a rough, cloudy, lumpy blend of clear and rime. It forms at the intermediate temperatures between the clear and rime regimes, roughly −10 to −15 °C, and in exactly the clouds the previous lessons flagged: deep nimbostratus and the cumulonimbus, whose depth spans several temperature bands at once. Its character combines the worst halves of its parents: rime's roughness with clear ice's weight and grip, often building fast and big — snowflakes frozen into the wet accretion can make it grow at spectacular rates.

Snow on the airframe

Snow itself is a special case with a clean rule: dry snow, falling at well-below-zero temperatures, does not stick to the airframe — there is no liquid to bind it, and it blows off the way it blew on. The danger appears near the freezing point: wet snow near 0 °C sticks, packing into a slushy layer that can refreeze on a sub-zero surface (on the ground as well as in flight), and snow met together with supercooled droplets freezes into the accretion, which is the mixed-ice route above. The exam's version of the rule: snow alone ices nothing unless it is wet, or unless liquid water is there to weld it on.

Hoar frost: ice without water

The fourth deposit needs no droplets at all. Hoar frost is a thin, white, crystalline coating formed by sublimation: water vapour turning directly to ice on a surface at or below 0 °C — no liquid phase, no cloud, no precipitation. It needs only the previous lesson's two clear-air situations: a surface radiatively cooled below the frost point on a clear, calm night on the ground, or a cold-soaked airframe descending into warm, moist air in flight. In aspect it is thin, feathery and light, and easily removed. But do not let the daintiness fool you: its crystalline roughness spoils the smooth flow a wing needs at takeoff, it obscures vision through a frosted windshield, and it must be removed before flight.

Which ice at which temperature

Put the three cloud-borne types on the previous lesson's temperature axis and the ladder almost draws itself:

clear ice — from 0 to about −10 °C;
mixed ice — from about −10 to −15 °C;
rime ice — from about −15 °C down to −40 °C.

Big wet drops at the warm end, small lean ones at the cold end, exactly as the supply curve demands. The boundaries are honest approximations, not fences: a cumulonimbus updraught can carry large drops far above the heights those bands suggest, delivering clear ice at temperatures where rime would be expected — one more reason the next lesson treats the big convective clouds with such respect. Hoar frost sits outside the ladder altogether: any sub-zero surface in clear, moist air will do.