Aircraft Icing: Reading the Warning Signs

Updated: 2026-09-27 · First published: 2026-09-27 · About this site

Airframe icing does not appear as a single code in a METAR the way a thunderstorm does. It has to be inferred: from the temperature, from the cloud, from the precipitation type, and from what a forecast says the atmosphere is doing a few thousand feet above the aerodrome. A few report groups are unambiguous — FZRA, FZDZ, FZFG — and the rest is reading the temperature structure. This guide covers both.

Where ice forms

Airframe ice needs supercooled liquid water: cloud droplets or raindrops that are below 0 °C but have not frozen. They freeze the moment something solid hits them — a wing leading edge, a propeller blade, a pitot tube. Three conditions must hold at the same time:

Visible moisture — cloud, fog, or precipitation. Clear air does not ice.
Temperature below 0 °C at the airframe (which, at speed, is slightly warmer than the OAT).
Temperature not so low that the droplets have already frozen: the practical band is 0 to about −15 °C, with the worst accretion between 0 and −10 °C.

Icing is therefore a layer: from the freezing level up to the top of the cloud or the −15 °C level, whichever comes first. It can be a few hundred feet thick in stratus or ten thousand in a cumulonimbus.

The types

Type	Forms from	Typical temperature	Character
Rime	Small droplets in stratiform cloud	−10 to −20 °C	Rough, white, on leading edges; spoils the aerofoil and blocks intakes
Clear (glaze)	Large droplets, freezing rain, cumuliform cloud	0 to −10 °C	Transparent, dense, flows back over surfaces beyond the de-icing boots; heavy and hard to see
Mixed	Cloud with a range of droplet sizes	−5 to −15 °C	Both at once; the usual serious case

The unambiguous codes

FZRA — freezing rain. Rain falling through a sub-zero layer near the surface: large supercooled drops, clear ice at the fastest rate aviation knows. A METAR with FZRA is a no-go for most aircraft and a severe icing SIGMET trigger. It implies a warm layer aloft (rain formed as liquid above) over a cold layer at the surface — the classic warm-front-over-cold-air structure.
FZDZ — freezing drizzle. Smaller drops, same mechanism, still serious because drizzle drops are large by cloud standards and the layer can be thick.
FZFG — freezing fog. Rime on the ground and in the first few hundred feet. The hazard is mostly to the aircraft that sat outside all night and to the climb-out through the fog layer.
PL / GS — ice pellets / snow pellets. Ice pellets are frozen raindrops, and their presence means there is freezing rain aloft: the drops froze on the way down through a cold layer. PL at the surface is a warning of FZRA a thousand feet up.
SN with temperature near 0 — wet snow, which sticks to the airframe and is a taxi and take-off contamination problem more than an in-flight icing one.

Reading the structure from a METAR

EDDM 271050Z 04008KT 2500 -RA BR OVC008 01/00 Q1019

Rain, not freezing rain, at +1 °C. Nothing in the weather group says ice. But the cloud is overcast at 800 ft and the surface is 1 °C: at a lapse rate of 2 °C per 1,000 ft the freezing level is about 500 ft above the ground, so the cloud base is barely above it and the whole cloud layer is below 0 °C. An aircraft climbing out enters supercooled cloud at 1,300 ft AAL and stays in it until it tops the layer. With a spread of 1 °C the cloud is wet. This is a moderate icing forecast that no code announces, and it is a common winter morning at EDDM Munich, LOWW Vienna or LTAC Ankara. Every airport page on this site shows the freezing level and the cloud layers together on the 3D profile and the meteogram, so the overlap that produces icing is visible rather than calculated.

Reading the structure from a forecast

Model forecasts supply what the METAR cannot: the temperature at each level. On the meteogram the freezing level is drawn as a line; the cloud layers as bands; the icing band is where a band sits between the 0 °C and roughly −15 °C lines with high relative humidity. Two structures deserve attention:

Warm layer aloft. Temperature rising with height through a frontal surface, above a sub-zero surface layer. Precipitation formed as rain above freezes on the way down: ice pellets, then freezing rain as the cold layer thins. The TAF may say PL or FZRA; the meteogram shows the temperature profile that produces it.
Stratocumulus under an inversion. A capped layer of cloud between 2,000 and 6,000 ft at −3 to −8 °C, full of supercooled water because there is nothing above to seed it with ice crystals. Long-lasting, uniform, moderate rime — and the layer a light aircraft has to climb through to reach the clear air on top.

Practical rules

Any FZRA, FZDZ or PL in a METAR or TAF on the route or at the destination: do not fly there without certified ice protection, and treat it as severe even with it.
Cloud with a base near or above the freezing level and a top you cannot see: assume icing in the layer. Know the top from the model profile before entering.
Pitot heat on before entering visible moisture below +5 °C. A frozen pitot fails silently.
Have an exit: a warmer level (descend below the freezing level over low terrain) or a colder one (climb above −15 °C or above the cloud). Deciding which before entering the cloud is the whole plan.
On the ground, clean the aircraft. Frost the thickness of sandpaper on the upper wing surface costs a measurable fraction of lift.

Frequently asked questions

What temperature range gives the worst icing?

Roughly 0 °C to −15 °C in cloud, with the most severe accretion between 0 and −10 °C where supercooled droplets are large and plentiful. Below about −20 °C most cloud water has already frozen to ice crystals, which do not stick. The band is in cloud or precipitation; clear air at −5 °C does not ice an airframe.

Why is freezing rain so dangerous?

Because the drops are large, supercooled and fall in clear air below the cloud, so the aircraft picks up clear ice quickly, over the whole airframe and behind the protected leading edges. FZRA in a METAR is a stop for aircraft without full anti-ice capability, and severe icing for those with it.

Does FZFG cause icing?

Yes — rime ice on the ground and in the climb through it, on the airframe, the pitot tube and the windscreen. It is a light-aircraft hazard mostly at the aerodrome: a pre-flight in freezing fog leaves a wing that will not fly cleanly.

Can I get icing above the freezing level shown on the meteogram?

The freezing level is where the temperature crosses 0 °C; the icing band is the cloud above it up to about −15 °C. So icing is expected above the freezing level, not below it, and its severity depends on the liquid water in the cloud there. An inversion can create a second band.

What is the difference between rime and clear ice?

Rime forms from small supercooled droplets freezing instantly on impact: rough, opaque, brittle, mostly on leading edges, at colder temperatures. Clear ice forms from large droplets that spread before freezing: smooth, transparent, heavy, flowing back over the wing, at temperatures near 0 °C. Mixed ice is both, and is the most common serious case.