PIREP Icing Codes: RIME, CLR, MIXED and the Intensity Ladder

PIREP icing codes sit in the /IC field, and they answer a question no forecast can: is there actually ice in that cloud right now, and how much.

Icing forecasts are cautious and broad by necessity. A PIREP is one aircraft, one place, one moment, and that specificity is what makes it worth reading.

The PIREP icing codes, decoded

The /IC field carries an intensity, usually a type, and often an altitude band. These are the intensity values the wxdecoder parser reads.

The type qualifier says which kind of ice formed.

Frequency qualifiers from the turbulence field can appear here too, so OCNL and CONS mean the same things they mean in /TB.

What each intensity actually means

The icing ladder is defined by what your ice protection can do about it, which makes it more useful than a thickness measurement.

Trace icing is perceptible. The rate of build is barely above the rate at which it sublimates away. It is not a hazard unless you sit in it for over an hour without protection.

Light icing may be a problem if you stay in it for over an hour. Occasional use of deicing equipment removes it. Changing level or heading solves it.

Moderate icing means even a short exposure is a problem. The rate is high enough that ice protection must be used, and diversion or a level change is called for.

Severe icing is the one with a hard definition. The rate of build is more than the equipment can handle. Protection no longer works, so an immediate change of altitude or heading is required. It is not a matter of comfort.

Why the ice type changes the reading

Two reports of moderate ice can mean quite different things depending on the type word beside them.

Rime forms from small droplets that freeze on impact, trapping air. It is white, rough and brittle, and it breaks off boots reasonably well. You can usually see it building.

Clear ice forms from large droplets that spread before they freeze. It is dense, transparent, and it bonds hard. Boots can flex underneath it without shedding it, and it runs back behind the protected surfaces where nothing can reach it.

So MOD CLR is a more serious report than MOD RIME at the same intensity. MIXED should be read as behaving like clear, because the clear part sets the difficulty.

The altitude band is the useful part

An icing PIREP without altitudes is a warning. One with altitudes is a plan.

/IC MOD RIME 060-080 says the ice was met between 6000 and 8000 feet. That tells you a climb of two thousand feet from the base of the layer clears it, or a descent of the same.

Where the report gives cloud tops in the /SK field as well, the picture is better again. Tops at 9000 with ice from 6000 to 8000 means climbing works and you know exactly how far.

Without that, you are guessing which way is out. Ice thins going up when the tops are near, because the water has already frozen into crystals. It thins going down when there is warm air below. Which is right depends on the day.

How to weigh somebody else's icing report

Three things change what a report means for you, and all three are in the PIREP.

Aircraft type, from /TP. A large jet with hot wings and a small piston twin with boots meet the same cloud differently. A jet reporting light ice tells you little about what a light aircraft will collect.

Time, from /TM. Icing conditions change with the freezing level and the cloud, so an hour-old report is history rather than news.

Position, from /OV. Icing is often tied to terrain and to fronts, so a report forty miles upwind may be more relevant than one twenty miles crosswind.

Icing in a forecast, for comparison

It helps to know how the same hazard is written when nobody has flown it yet.

A civil TAF has no icing group at all. Icing reaches you through AIRMET Zulu and through SIGMETs, as an area and an altitude band rather than a code.

A military TAF does carry one. It uses a six-digit group where a single digit gives both the type and the intensity, so 620065 and 510009 are complete icing forecasts. That numeric scale maps onto the same ladder as the words here, which is why a trace in one product reads as a trace in the other.

The practical difference is confidence. A forecast says ice is possible in a volume of air. A PIREP says an aircraft met it, at a time, at a height.

Verdict on PIREP icing codes

Read the /IC field as intensity first, type second, altitude band third. The intensity tells you whether your equipment can cope. The type tells you how hard the ice will be to shed. The band tells you the way out.

And treat NEG as a real report. In an area forecast full of cautious icing warnings, a pilot who flew the layer and found nothing is giving you the most current information available.

Last updated August 29, 2026