Structural icing and the freezing level

Ice needs two things at once: visible moisture, and a surface below freezing. Everything about which kind you get comes down to how big the droplets are.

Weather Information — FI.III.C · CA.I.C · PA.I.C

Frost first, because it is not icing

On a cool clear night the ground cools the air touching it below its dewpoint. Moisture deposits on surfaces — as dew above freezing, as frost below it. Frost is not structural ice and it does not add meaningful weight, which is exactly why it gets dismissed. It is dangerous for a different reason: it disrupts the smooth flow of air over the wing, and a wing that cannot make its designed lift will not fly off the runway it usually uses.

The two conditions

Structural icing needs visible moisture and a surface at or below freezing, together. Either alone gives you nothing. That is why the freezing level is the number a briefing turns on: it tells you the altitude at which cloud stops being merely cloud.

The droplets are supercooled — still liquid below 0 °C — and freeze on contact with the airframe. What decides the kind of ice is how quickly they freeze after they hit.

Clear, rime and mixed

Clear ice comes from large droplets that do not freeze instantly. They flow back along the surface before they solidify, so the ice is smooth, glassy, heavy and hard to see. It bonds firmly and follows the aerofoil shape, which means it can build a long way before it is obvious.

Rime ice comes from small droplets that freeze on impact, trapping air between them. It is opaque, white, brittle and rough — and that roughness is the point: it wrecks the boundary layer while adding relatively little weight.

Mixed is what you usually actually get, because real cloud contains a range of droplet sizes.

Freezing rain tells you about air you cannot see

Freezing rain at the surface means there is a deep above-freezing layer aloft with a shallow below-freezing layer underneath it. The rain fell as liquid through the warm layer and became supercooled on the way down.

Read what that implies for an aircraft: climbing usually takes you into warmer air. It is one of the few situations where the instinct to climb out of the weather is right for a reason most people cannot state — and where descending into the shallow cold layer is exactly wrong.

How it is reported

AIRMET Zulu carries icing and the freezing level. Sierra covers IFR and mountain obscuration, Tango covers turbulence and strong surface winds — the three letters are worth knowing cold, because they are how the whole system is indexed.

PIREPs matter more for icing than for almost anything else. Icing intensity is a reported quantity: trace, light, moderate, severe describe what an aircraft actually accumulated, and no model output substitutes for someone who was just there in something like your aeroplane.

What is in force right now

Keep going

• Thunderstorms
• Turbulence, shear and mountain waves
• Fog and obstructions to visibility — freezing fog lives here too
• PIREPs, AIRMETs, SIGMETs and TFRs

Sources

Icing types, formation and the freezing-rain mechanism are from the FAA Aviation Weather Handbook (FAA‑H‑8083‑28); AIRMET Zulu issuance is in the AIM, chapter 7.