Freezing mist is absent from METAR reports because it has no present-weather code. The FAA’s surface observing order and the NWS manual define mist as a visibility obscuration, not a freezing-precipitation type. So neither ASOS nor AWOS can report it automatically, and no current roadmap adds that capability.
The only reliable confirmation is direct human observation — crews and observers looking at the actual wing and ramp rather than trusting the printed report.
It’s a few degrees below freezing, and the METAR says BR — mist, visibility around six miles, nothing that would pause a departure. What the report doesn’t say is that the mist is supercooled, cold enough to freeze on contact with a wing.
A freezing mist METAR doesn’t exist. The automated station that generated that report was never programmed to look for it.
That’s the operational hole this article picks apart. Freezing mist can form at 0 °C or below, can coat an airframe, and can stay entirely invisible in the data pilots are trained to trust.
The fix isn’t in the sensor — it’s in direct human observation, a practice automation was supposed to make optional.
No study has put a number on how often freezing mist slips past the network. But the adjacent failure shows the pattern clearly enough, and it bends deicing decisions downstream. The open question is whether that gap is temporary or built into the system’s grammar.
Why freezing mist never gets its own METAR line
Start with the rulebook, because the rulebook writes the report. In FAA and NWS surface-weather guidance, mist is an obscuration — a visibility range plus a small temperature–dewpoint spread — not a precipitation type.
That’s not a semantic quibble. It decides which codes exist.
ASOS can generate BR when visibility sits between roughly 5/8 and 7 statute miles with the air near saturation. Freezing rain and freezing drizzle get their own present-weather codes. Freezing mist doesn’t.
The reportable present-weather list has no entry for it, so neither automation nor an observer can file it.
So mist at 0 °C or below falls straight through the language of the report. A station prints BR and stops, while a thin accretion pattern can already be forming on a cold wing.
The absence isn’t a sensor failure — it’s built into the reporting system’s vocabulary.
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What ASOS and AWOS actually detect
ASOS and AWOS were built to reduce the need for human observers, and they’ve mostly done that. More than 900 stations now cover the United States; certified humans remain at roughly 130 airports where observations matter most. The trade is that automation is only as sharp as its algorithms, and those algorithms are uneven across winter conditions.
Freezing drizzle is the clearest case — current ASOS cannot place it in a METAR automatically. ASOS freezing mist detection is even more fundamental: there’s no code to trigger.
When no certified observer is on duty, reports are marked AUTO and can’t be corrected in real time. A station can read BR at -2 °C all morning and leave nobody to ask whether that mist is accreting.
How often freezing mist goes unreported versus other icing conditions not reported is a question no primary source has answered. That’s an unresolved data gap, not evidence the problem is rare. The table maps the asymmetry side by side.
| Phenomenon | METAR code or description | Automatic ASOS/AWOS reporting capability | Human observer augmentation needed |
|---|---|---|---|
| Mist (BR) | Visibility between about 5/8 and 7 statute miles with a small temperature–dewpoint spread | Yes — automatic BR from visibility and dewpoint-spread thresholds | Observer may clarify associated icing in remarks; not required for BR itself |
| Fog (FG) | Fog reducing visibility below about 5/8 statute mile | Yes — automatic FG when visibility drops below fog threshold | Observer may initiate SPECI for freezing fog or significant changes |
| Freezing fog (FZFG) | Fog or mist below 0 °C with ice accretion risk | Partially — automated code exists but misreported with snow in automated data | Yes — SPECI at onset/end; observers confirm before freezing fog holdover times |
| Freezing rain (FZRA) | Rain at sub-zero temperatures that freezes on contact | Yes — ASOS detects via present-weather, icing sensor, and temperature thresholds | Observers take SPECI at onset/end and correct errors |
| Freezing drizzle (FZDZ) | Drizzle at sub-zero temperatures that freezes on contact | No — current ASOS cannot automatically report it | Yes — staffed observers report FZDZ manually; unstaffed sites cannot |
| Freezing mist (no dedicated code) | Mist at 0 °C and below, causing ice accretion while METAR only shows BR | No — not a distinct METAR code; ASOS/AWOS cannot report it automatically | Yes — best confirmed by observation; inferred from BR at freezing temperatures |
| Snow (SN) | Frozen precipitation reaching the surface | Yes — ASOS reports intensity via visibility and present-weather sensor | Observers may augment mixed conditions or correct misclassified obscurations |
| Ice fog (no unique METAR code separate from FZFG) | Very dense freezing fog typically in extremely cold conditions | No separate automatic category — treated within freezing fog/fog framework | Yes — observers describe ice fog and icing hazards in remarks |
| Source: Federal Aviation Administration (FAA); National Weather Service (NWS); Transport Canada | |||
Scan the automatic column. Freezing rain gets a yes; freezing drizzle gets a no; freezing mist sits outside even that comparison, with no code to hang on it.
What a misread costs at the deicing pad
The number that matters here is 85%. In an analysis published in the American Meteorological Society’s Journal of Applied Meteorology and Climatology, U.S. automated stations paired freezing fog with snow incorrectly 85% of the time.
That mismatch makes holdover times shorter than they should be. A shorter holdover time means a second spray, another delay, another round of gate chaos.
It’s the same report-and-ramp disagreement that freezing mist creates. A METAR may say snow with freezing fog while the cockpit and ramp conditions disagree. Crews then file PIREPs and voice reports to reconcile what the sensors missed — before anyone locks in an aggressive holdover table.
Here’s the scene that sticks. A passenger on the jet bridge can watch a deicing truck working through freezing mist, while the METAR at the same airport reads only BR.
Transport Canada’s ground-icing guidance isn’t surprised by this — it groups freezing mist with freezing fog and ice crystals in the holdover column. The report stays clean. The wing doesn’t.
How the rulebook locked the blind spot in
The deeper reason is that the report’s grammar has no slot for this. FAA Order JO 7900.5E, section 9.14, tells observers to file a SPECI when freezing rain or freezing drizzle begins, ends, or changes intensity.
It asks no such thing when ordinary mist starts to freeze on contact. NWS Observing Handbook No. 8 treats mist as a visibility-and-dewpoint obscuration, not an icing phenomenon, so the automation never tests for supercooled droplets inside that BR code.
The result is a category error that goes back to the observation desk. ASOS can mark BR, but there is no sensor chain that asks the follow-up: is this mist cold enough to accrete? That follow-up is left to a person on the ramp, a pilot’s PIREP, or a deicing crew’s judgment.
One automation-impact study ties unreported freezing drizzle to a crew continuing an approach — the same hole, one condition over.
The open question is whether newer sensors could close it. According to NOAA lifecycle material through 2024, no ASOS modernization roadmap adds automated freezing mist detection. That makes observation a design requirement, not a stopgap.
What this means at the gate
For a passenger, the freezing mist gap translates into two things. A flight can be delayed or sent for a second deicing even when the airport’s reported weather looks completely benign, because the report a crew read didn’t carry the condition that actually mattered. And the fix for that gap is human — a ramp agent who sees ice, a pilot who files a PIREP, an observer who writes a remark.
That’s why the age of automation hasn’t retired manual weather observation in aviation. At the airports where it matters most, certified observers still stand on the ramp and correct what the machine can’t classify. If you fly into a U.S. airport on a near-freezing morning and the METAR says only BR, the safest assumption is that the report is incomplete, not that the air is clean.
Sensor upgrades are measured in years, not seasons. Until an algorithm can ask the supercooled question, observation will keep doing the work no machine has been assigned.
Questions? Answers.
Why is freezing mist missing from METAR reports even at automated stations?
Freezing mist has no dedicated present-weather code in FAA Order JO 7900.5E or NWS Observing Handbook No. 8, which treat mist only as a visibility and dewpoint obscuration. Even an automated station producing a full METAR never reports freezing mist — it shows BR — and the condition must be confirmed by observation.
Can ASOS or AWOS automatically detect freezing mist?
No. Current ASOS cannot automatically report freezing drizzle either, and no modernization roadmap through 2024 adds automated freezing mist detection. The guidance says freezing mist is never reported by METAR and is best confirmed by observation.
How often do automated systems misreport freezing conditions that affect deicing?
U.S. automated stations misreported freezing fog paired with snow about 85% of the time in an AMS Journal of Applied Meteorology and Climatology analysis. That shorter-than-reality pairing drives shorter holdover times and repeat deicing.
Are sensor upgrades planned that would let ASOS report freezing mist automatically?
No current plan exists. NOAA and FAA lifecycle material through 2024 shows an Enhanced Precipitation Indicator and a freezing drizzle algorithm were tested but rejected for failing FAA standards, and no roadmap adds automated freezing mist detection.
