Contrail Forecast Tool
This tool uses public weather data to estimate whether aircraft contrails are likely to form and persist at typical cruising altitudes on a given day.
About the Results
How to Interpret the Charts
Each chart shows how favourable the atmosphere is for contrails above your chosen location on the selected day.
- X-axis (left to right): hour of day (local time)
- Y-axis (bottom to top): cruising altitude in thousands of feet (for example, 30k = 30,000 ft)
- Each dot: one hour at one altitude band
- Colour / score (0–4): how likely contrails are to form and persist at that time and height
What the Scores Mean
- 0 – Contrails unlikely: conditions are not favourable for contrails to form.
- 1 – Brief contrails possible: may form but likely fade quickly.
- 2 – Mostly short-lived: marginal persistence (RHi 90–100).
- 3 – Persistent likely: more supportive of longer-lasting contrails (RHi 100–110).
- 4 – Very persistent likely: strong ice-supersaturation, higher chance of long-lived trails and spreading (RHi ≥ 110).
In practice, 3–4 are the conditions most consistently noticed from the ground because contrails tend to last longer.
You may notice the SAC chart looks broadly similar to the Basic chart, but with more “1” dots. That is expected. Both charts use the same weather-model data and the same persistence scoring; the only difference is the formation test. The SAC method is often able to pass formation in a wider set of cold cruise-level conditions than the Basic fixed thresholds. When those additional cases are not ice-supersaturated (low RHi), the tool still classifies them as 1: contrails could form briefly, but are unlikely to persist. In other words, the SAC chart is picking up more marginal, short-lived contrail opportunities, not predicting dramatically more long-lived trails.
What is a flight level?
A flight level (FL) is an aviation way of expressing altitude.
For example:
-
FL300 means 30,000 feet (approximately)
The tool evaluates typical jet cruise bands from about 27,000 to 44,000 feet, shown as 27k–44k on the vertical axis.
How the Calculations are Made
Both charts use the same hourly upper-air weather model data (temperature, humidity and geopotential height) from Open-Meteo’s GFS model. For each hour, the tool interpolates the model data to the flight levels shown on the chart.
Then it applies a two-step logic:
- Formation: can a contrail start at this hour/altitude?
- Persistence: if it starts, is the air moist enough (with respect to ice) for it to last?
Persistence is estimated using RHi (relative humidity with respect to ice), derived from the model humidity. The score 1–4 is based on the RHi range (bins). Flight levels are a pressure-altitude convention; using geometric height for interpolation is an approximation. Geopotential height is above mean sea level, not height above ground.
Why there are two charts
The charts differ only in the formation step:
Chart 1: Simple Temperature/Humidity Thresholds
This uses a simple, transparent rule: if the air is cold enough and humid enough, formation is considered possible. Persistence then uses the same RHi scoring as above.
Chart 2: Schmidt–Appleman Criteria (SAC)
This uses a standard contrail formation model known as the Schmidt–Appleman criterion (SAC). SAC depends on both the atmosphere and aircraft/engine properties, which are not known for every flight overhead. To make it usable as a general forecast, the tool uses typical jet assumptions, including:
- Jet fuel (kerosene/Jet-A)
- A representative water vapour emission index (amount of water produced per fuel burned)
- A typical cruise propulsion efficiency
- Standard thermodynamic constants (e.g., heat capacity of air)
Those assumptions are applied consistently so you can compare how a physics-based formation test behaves relative to the simpler threshold method, using the same weather data and the same persistence scoring.
They are intended to represent a typical modern commercial jet at cruise, so they broadly fit the large population of short- to long-haul passenger aircraft flown by the major manufacturers.
In practical terms, the assumptions are most representative of aircraft in these families:
- Airbus A320 family (A319/A320/A321, including neo variants)
- Boeing 737 family (NG and MAX)
- Airbus A330 / A350
- Boeing 767 / 777 / 787
They also broadly cover many regional jets with similar cruise altitudes and turbofan engines, though smaller aircraft and older engine designs can deviate more.
Important note on real-world visibility
These charts show atmospheric favourability, not whether aircraft will be present overhead. What you see from the ground also depends on air traffic, routing, sun angle, haze, and thin background cloud.