Multi-Model Icing Analysis
PlaneWX goes far beyond traditional AIRMETs and PIREPs. We analyze icing conditions using three independent weather models, giving you confidence levels, cloud layer boundaries, ice type forecasts, and total exposure estimates — all tailored to your route and aircraft.
Why Multi-Model Icing Matters
Traditional icing information comes from two sources: G-AIRMETs (broad geographic areas with blanket severity ratings) and PIREPs (sporadic pilot reports that may be hours old). Both have significant limitations:
G-AIRMETs: Too Broad
- • Cover hundreds of thousands of square miles
- • Single severity for the entire area
- • No information about specific altitudes within the range
- • Don't tell you where the clouds actually are
- • Updated every 3 hours — conditions change faster
PIREPs: Too Sparse
- • Depend on other pilots filing reports
- • Subjective severity assessments
- • Represent a single point in space and time
- • Often hours old by the time you see them
- • Huge gaps in coverage, especially at night and in rural areas
PlaneWX's approach: Instead of relying solely on these broad products, we query three independent numerical weather models at multiple points along your specific route. Each model provides temperature, humidity, and cloud data at twenty altitude levels — giving us a detailed vertical profile of where icing conditions actually exist, how severe they are, and how confident we are in the forecast.
The Three Models
PlaneWX queries three weather models independently — two from NOAA and one from ECMWF. Each has different strengths, and comparing their output gives us confidence scoring — something no single model can provide alone.
High-Resolution Rapid Refresh
NOAA's highest-resolution hourly model — the gold standard for short-range icing forecasts.
3 km
Every hour
0–18 hours
Cloud layers, convection, terrain effects
HRRR's 3-km grid captures terrain-induced effects that coarser models miss — critical for mountain flying where icing conditions can vary dramatically over short distances.
Global Forecast System
NOAA's primary global model — the backbone of most weather predictions.
~13 km
4 times daily
0–16 days
Big-picture trends, extended-range icing outlook
GFS provides the longest forecast range. When HRRR data expires (beyond 18h), GFS and ECMWF continue to provide icing guidance — critical for PlaneWX's mission of long-range trip planning.
European Centre for Medium-Range Weather Forecasts
Widely regarded as the world's most accurate global forecast model.
~9 km
4 times daily
0–10 days
Overall accuracy, moisture fields, jet stream patterns
ECMWF's IFS model consistently outperforms other global models in verification studies. Its independent European perspective on atmospheric conditions strengthens consensus scoring alongside the two NOAA models.
Why Three Models?
No single model is always right. By comparing three independent models (HRRR, GFS, and ECMWF), PlaneWX determines how much to trust the forecast. When all models agree, confidence is HIGH. When they disagree, confidence drops — and that disagreement itself is valuable information. It tells you the atmosphere is uncertain and conditions could go either way.
How We Detect Icing
Icing requires two ingredients simultaneously: subfreezing temperatures and visible moisture (clouds). PlaneWX checks both at each altitude level along your route.
Temperature Check
PlaneWX detects icing across two zones: a primary zone (0°C to -20°C) where ~90% of structural icing occurs, and an extended zone (-20°C to -40°C) where supercooled water can persist in convective updrafts and orographic lift. Below -40°C, homogeneous nucleation ensures all water is frozen — liquid water cannot exist.
0°C to -10°C: Maximum icing risk — supercooled water droplets most common (clear ice zone)
-10°C to -20°C: Decreasing risk — mixed phase, smaller droplets (rime ice zone)
-20°C to -40°C: Extended range — supercooled water rare but possible in vigorous convection. Probability attenuates linearly toward -40°C
Below -40°C: No icing — homogeneous nucleation, liquid water cannot exist
Moisture Check
Relative humidity ≥ 70% at a pressure level indicates proximity to cloud or visible moisture. Higher RH means denser moisture and higher icing probability.
RH ≥ 90%: Almost certainly in cloud — highest icing probability
RH 80–89%: Likely in cloud — moderate-to-light icing probability
RH 70–79%: Near cloud threshold — trace icing probability (primary zone only)
RH < 70%: Dry air — no icing at this level
Icing Probability Formula
At each altitude level, PlaneWX calculates an icing probability from 0–100% based on:
- Is the temperature between 0°C and -40°C? If not, probability is 0% regardless of moisture.
- Is relative humidity ≥ 70%? If not, probability is 0% — insufficient moisture for icing.
- Primary zone (0°C to -20°C): RH ≥ 90% with temp above -10°C yields moderate severity (80–90% prob). Lower RH and colder temps scale down through light and trace.
- Extended zone (-20°C to -40°C): Only RH ≥ 80% triggers icing detection. Probability attenuates linearly with temperature — approaching zero at -40°C. Max severity is trace.
Severity Levels
PlaneWX converts the calculated icing probability into FAA-standard severity levels. These are the same severity terms used in AIRMETs, PIREPs, and aviation forecasts.
0–19%
NONE
No icing expected. Temperature and/or moisture conditions don't support ice accumulation.
20–39%
TRACE
Ice becomes perceptible. Rate of accumulation slightly greater than the rate of sublimation. Not hazardous unless encountered for more than an hour.
40–69%
LIGHT
Rate of accumulation may create a problem if flight continues for more than an hour. Occasional use of de-icing/anti-icing equipment removes or prevents accumulation. Non-FIKI aircraft should exit these conditions.
70–89%
MODERATE
Rate of accumulation requires frequent cycling of de-icing equipment or diversion. Even FIKI-equipped aircraft should have an exit strategy. Significant impact on aircraft performance — increased drag, reduced lift, higher stall speed.
90–100%
SEVERE
Rate of accumulation exceeds the capability of de-icing/anti-icing equipment. Immediate action required. Even FIKI-certified aircraft are not approved for severe icing. PlaneWX always marks severe icing as unfavorable regardless of aircraft equipment.
Multi-Model Confidence
One of PlaneWX's most powerful capabilities is multi-model consensus scoring. Instead of trusting a single model, we compare all three and rate how much they agree.
HIGH
Unanimous
All models agree on icing severity at this altitude and location. You can have high confidence in this forecast. If all three say moderate icing at 8,000 ft, expect moderate icing at 8,000 ft.
MODERATE
Majority
Most models agree, with one or two differing. The majority view is likely correct, but there's some uncertainty. PlaneWX uses the majority consensus as the primary forecast and notes the dissent.
LOW
Split
Models disagree significantly. This happens at atmospheric boundaries where conditions are changing rapidly. Low confidence doesn't mean no icing — it means the atmosphere is uncertain. Plan for the worse-case model and have an exit strategy.
Model agreement labels appear in your briefing's icing section and in the Weather Sources panel. "Unanimous" means all models agree. "Split" means they disagree. This is valuable intelligence that traditional aviation weather products simply don't provide.
Route Sampling
PlaneWX doesn't just check icing at your departure and arrival. We sample 3 to 7 points along your route, depending on route length, to build a complete picture of icing conditions from takeoff to landing.
At Each Sample Point, We Query:
- Temperature at 20 pressure levels (1000–300 hPa)
- Relative humidity at 20 pressure levels
- Geopotential height (actual altitude of each pressure level)
- Dewpoint depression at each level
- Cloud cover at low, mid, and high layers
- Freezing level height from each model
- Surface temperature and visibility
- Precipitation type and probability
This means for a typical 300 NM flight, PlaneWX analyzes approximately 300 data points (5 sample points × 20 altitude levels × 3 models) to build your icing profile. All of this data is available in the Weather Sources panel for full transparency.
Ice Types: Clear, Rime, and Mixed
Not all ice is created equal. PlaneWX identifies the type of ice you're likely to encounter based on the temperature at your cruise altitude. This matters because different ice types affect your aircraft differently and require different responses.
Clear IceMost Dangerous
0°C to -10°C
Smooth, transparent, glass-like
Large supercooled water droplets
Clear ice is heavy, hard to see, and difficult to remove. It forms when large supercooled droplets hit the aircraft and spread before freezing, creating a smooth layer that conforms to the airfoil shape. It accumulates rapidly, adds significant weight, and can change the wing's aerodynamic profile. De-icing boots may be less effective because the ice conforms tightly to the surface.
Rime IceMore Common
-10°C to -20°C
Rough, opaque, milky white
Small supercooled water droplets
Rime ice forms when small droplets freeze instantly on contact, trapping air bubbles and creating a rough, white, opaque coating. It accumulates on leading edges and is generally easier to remove with de-icing equipment. While rime ice is easier to manage than clear ice, it still degrades performance and should not be taken lightly.
Mixed Ice
-10°C to -15°C
Combination of clear and rime
Both large and small droplets present
Mixed ice combines characteristics of both clear and rime ice and can be unpredictable. It often forms in clouds with varying droplet sizes or at temperatures in the transition zone.
SLD Risk & Warm Nose Detection
Supercooled Large Droplets (SLD)
SLD conditions are among the most dangerous icing scenarios in aviation. Supercooled large droplets are raindrops or large cloud droplets that remain liquid below 0°C. When they strike an aircraft, they run back along the surface before freezing — accumulating ice behind the protected areas of boots, heated surfaces, and TKS panels.
Even FIKI-certified aircraft can be overwhelmed by SLD conditions. The American Eagle ATR-72 crash in Roselawn, Indiana (1994) was caused by SLD ice accumulating behind the de-ice boots. This accident led to significant FAA regulation changes.
What Is a "Warm Nose"?
A warm nose is a layer of above-freezing air sandwiched between subfreezing layers in the atmosphere. It's a key indicator of SLD risk and freezing rain.
Altitude profile (example):
15,000 ft -18°C ❄️ Subfreezing (ice crystals)
10,000 ft -8°C ❄️ Subfreezing (supercooled water)
6,000 ft +2°C 🌡️ WARM NOSE — ice melts into rain
3,000 ft -3°C ❄️ Subfreezing — rain refreezes as SLD/freezing rain
Surface -1°C ❄️ Subfreezing
When ice crystals or snow fall through the warm layer, they melt into rain. As that rain continues falling into the subfreezing layer below, it becomes supercooled — liquid water below 0°C. These large, supercooled drops are the defining characteristic of SLD conditions.
How PlaneWX Detects SLD Risk
PlaneWX scans the temperature profile from each model at each sample point along your route. If we detect a warm layer (above 0°C) sandwiched between subfreezing layers, we flag it as a warm nose and assess SLD risk. When a warm nose is detected, your briefing will explicitly call it out with an explanation of what it means for your flight.
Total Icing Exposure
PlaneWX calculates your total time in icing conditions broken down by flight phase. This is calculated server-side from your route, altitude, aircraft climb/descent performance, and the icing layer boundaries — not estimated by AI.
Climb
Time spent climbing through the icing layer from departure airport elevation to cruise altitude. Based on a standard 500 ft/min climb rate.
Cruise
Time at cruise altitude inside the icing layer. If your cruise altitude is within the icing band, this is the duration of your entire cruise segment.
Descent
Time spent descending through the icing layer from cruise altitude to destination elevation. Based on a standard 500 ft/min descent rate.
TKS Fluid Planning
If your aircraft uses a TKS (weeping wing) anti-ice system, total icing exposure is critical for planning fluid usage. Most TKS systems have limited fluid capacity — typically 1.5 to 4 gallons depending on the system. Knowing exactly how many minutes you'll be in icing helps you determine if you have enough fluid for the flight, or if you need to adjust your route or altitude.
Example: A TKS system consuming 0.6 gal/hr in "normal" mode with a 2.5 gallon tank provides approximately 4 hours of protection. If your total icing exposure is 45 minutes, you have plenty of margin. If it's 3 hours, you're cutting it close.
How Icing Affects Your WX Score
Icing deductions use your personal minimums and aircraft capabilities. The same icing conditions can produce very different scores for different pilots — and that's by design.
Soft / Hard Limit System
PlaneWX uses a two-tier minimums system for icing:
Comfort (Soft Limit)
The icing severity you're comfortable with. Exceeding this puts you in the caution zone with a graduated penalty.
Example: Soft = Light. If forecast shows Light icing, minimal deduction.
Limit (Hard Limit)
The maximum icing severity you'll accept. Exceeding this is unfavorable — score drops to 0% within 12 hours.
Example: Hard = Moderate. If forecast shows Severe icing, unfavorable.
| Scenario | Impact | Example |
|---|---|---|
| Any icing, no FIKI | Unfavorable (0%) | Light icing forecast, aircraft has no de-ice/anti-ice |
| Severity exceeds hard limit | Unfavorable (0%) | Hard limit = Moderate, forecast = Severe |
| Severe icing (any pilot) | Unfavorable (0%) | Severe icing is always unfavorable regardless of FIKI or limits |
| Severity between soft and hard | Caution (-5 to -25) | Soft = Light, Hard = Moderate, forecast = Moderate |
| Severity at or below soft limit | Minimal (-5) | Soft = Moderate, forecast = Light (within comfort zone) |
Aircraft-specific overrides: If your aircraft profile has icing limits set, those override your personal minimums (using the more restrictive value). A FIKI-equipped aircraft with icing hard limit set to "Moderate" will use that limit. A non-FIKI aircraft will always be flagged unfavorable for any icing — even trace.
Reading the Icing Section in Your Briefing
The icing section of your briefing contains several key pieces of information, all derived from the multi-model analysis. Here's how to read it:
Icing Layer Boundaries
Your briefing shows the altitude range where icing conditions exist — for example, "Icing Layer: 853 – 14,101 ft". This tells you exactly where you'll enter and exit icing on climb and descent. If your cruise altitude falls within this range, you'll be in icing for the entire cruise.
Cloud Entry / Exit Altitudes
Based on relative humidity profiles, PlaneWX identifies where you'll likely enter and exit clouds. Cloud base and tops are derived from the altitude where RH crosses the 80% threshold. This is especially useful for VFR pilots assessing cloud clearance requirements.
Freezing Level
The altitude where the temperature crosses 0°C. Each model provides its own freezing level, and your briefing shows the consensus. If the freezing level is near or below your departure airport elevation, expect icing from the surface up.
Confidence & Model Agreement
Each briefing includes the multi-model confidence level (HIGH / MODERATE / LOW) and whether models are "unanimous" or "split" at your cruise altitude. This helps you gauge how much to trust the icing forecast and whether conditions might be better or worse than predicted.
Ice Type at Cruise
Based on the temperature at your cruise altitude, PlaneWX predicts whether you'll encounter clear ice, rime ice, or mixed ice. Hover or tap on these terms in your briefing for educational tooltips explaining each type.
Total Icing Exposure
The total time you'll spend in icing conditions, broken down by climb, cruise, and descent. This is computed from your aircraft's performance data and the icing layer boundaries — not estimated by AI. Use this for TKS fluid planning and assessing whether prolonged exposure is within your aircraft's capabilities.
Full Transparency: Weather Sources Panel
PlaneWX believes in showing its work. Every briefing includes a Weather Sourcespanel where you can see the raw model data that powered the icing analysis.
What You'll Find in the Forecasts Tab
- Model overview: Which models were queried (HRRR + GFS + ECMWF), overall confidence, max severity, and icing layer range
- Per-sample-point tables: Each sample point along your route shows a table with altitude, temperature, relative humidity, icing probability, severity, and model agreement
- Freezing levels per model: HRRR, GFS, and ECMWF freezing levels shown side by side at each sample point
- Cloud highlighting: Altitude levels with RH ≥ 80% are highlighted to show where you're likely in clouds
- Fetch timestamp: Exactly when the model data was retrieved, so you know how current it is
Why do we show raw data? Most aviation weather apps present conclusions without evidence. PlaneWX shows you exactly what data we used and how we arrived at our assessment. This builds trust, enables independent verification, and helps you become a better weather decision-maker over time. If you disagree with our assessment, you can see the underlying data and make your own judgment.
Section-Level Sources
Every section of your briefing — icing, turbulence, ceilings, winds — has a Sources button. Tapping it reveals exactly which weather products contributed to that specific section, along with raw data snippets.
Icing Section Sources Include:
- HRRR3-km resolution model sounding at sample points along your route
- GFS13-km global model sounding at sample points along your route
- G-AIRMETActive G-AIRMET Zulu (icing) advisories along your route
- PIREPsRecent pilot reports of icing within your route corridor
Educational Tooltips
PlaneWX is designed to make you a better weather decision-maker, not just tell you what to do. Throughout your briefing, you'll see aviation terms and model names that are interactive — hover or tap to see educational definitions.
Terms with Tooltips:
These terms appear underlined with a dotted border in your briefing. Hover (desktop) or tap (mobile) to see the full definition without leaving your briefing.
How This Compares
PlaneWX's multi-model icing analysis provides significantly more detail and transparency than traditional aviation weather tools.
| Capability | PlaneWX | Traditional |
|---|---|---|
| Multi-model comparison | 3 models | Single product |
| Confidence scoring | Yes | No |
| Route-specific sampling | 3–7 points | Broad areas |
| Cloud layer boundaries | Yes (RH-derived) | Not provided |
| Ice type prediction | Clear/rime/mixed | Not provided |
| SLD / warm nose detection | Yes | SLD AIRMETs only |
| Total icing exposure | Climb/cruise/descent | Not calculated |
| Personal minimums integration | Soft + hard limits | Generic severity |
| Raw data access | Full transparency | Conclusions only |
| Altitude-specific assessment | 20 pressure levels | Altitude range only |
Limitations & Important Notes
Model Data Is a Forecast
Model soundings are forecasts, not observations. They predict where icing conditions will be, not where they are right now. Real-time PIREPs and current METARs always take precedence for imminent flights. PlaneWX includes both in your briefing.
Severity Is Derived, Not Measured
Icing severity is derived from temperature and humidity profiles using proven meteorological relationships. It is not a direct measurement of ice accumulation rate on your specific aircraft. Actual accumulation depends on aircraft speed, airfoil shape, droplet size, and exposure time.
HRRR Coverage Is U.S. Only
The HRRR model covers the contiguous United States only. For flights outside CONUS, the analysis falls back to GFS and ECMWF (two-model consensus). Confidence scoring adjusts accordingly.
Not a Substitute for Official Briefing
PlaneWX's multi-model icing analysis supplements — but does not replace — official weather products. Always cross-reference with current G-AIRMETs, SIGMETs, and PIREPs. The pilot in command is always responsible for the final go/no-go decision per 14 CFR 91.3.
About This Documentation
PlaneWX's multi-model icing analysis is continuously improved based on pilot feedback and model verification. The system currently uses HRRR, GFS, and ECMWF models via the Open-Meteo API, which provides access to NOAA and ECMWF operational weather model data.