Learn to read aviation prognostic weather charts, including SIGWX, fronts, isobars, jet streams, hazards, flight levels and valid times.
Read an aviation prognostic weather chart by checking its chart type, geographic coverage and valid time first, then interpret the forecast systems and hazards using that chart’s legend. Pressure charts show fronts, highs, lows and isobars; SIGWX charts add operational hazards such as turbulence, icing, thunderstorms, jet streams and tropopause levels.
For Aviation & Real-World Flying study and realistic simulator planning, treat every prognosis chart as a time-stamped forecast rather than a live weather picture. Symbols, altitude ranges and hazard thresholds vary between chart producers, so the legend printed on the specific chart always takes precedence.
What is the difference between SIGWX and pressure charts?
A pressure chart describes the broad weather system, while a significant weather chart shows the hazards that system is forecast to produce.
| Chart type | Main information | Best use | What it does not show well |
|---|---|---|---|
| Surface pressure prognosis | Mean sea-level pressure, highs, lows, isobars, fronts and troughs | Understanding system movement, wind patterns and likely weather changes | Exact airport ceiling, visibility or runway wind |
| Low-level SIGWX | Depending on the product: cloud, precipitation, visibility, thunderstorms, icing, turbulence and freezing level | Departure, arrival and lower-altitude route planning | Hazards above its stated vertical limit |
| High-level SIGWX | Cumulonimbus areas, clear-air turbulence, jet streams, tropopause information and selected major hazards | Upper-level cruise planning | Low cloud, surface visibility and most low-level icing |
Do not assume that every chart labelled SIGWX covers the same altitude band. Read the title block: the chart may be low-level, medium-level or high-level, with its lower and upper limits stated as flight levels.
How do you read a prognostic weather chart step by step?
The reliable method is to establish when and where the forecast applies before interpreting any weather symbol.
- Identify the product. Read the title, issuing region, geographic coverage and altitude range. Confirm whether it is a pressure prognosis, low-level SIGWX or high-level SIGWX chart.
- Find the valid time. Do not confuse the issue time with the time for which the forecast is valid. A group such as
150000Zmeans the 15th day of the month at 0000 UTC; some products instead show a forecast lead such asT+24. - Check whether validity is a point or period. A chart valid at 1200 UTC is a forecast snapshot. A chart covering a stated interval describes conditions expected during that period.
- Plot the route mentally. Locate the departure, destination, alternates and major route segments. Note which systems or hazard areas intersect them rather than reading the chart as one regional summary.
- Build the pressure picture. Follow the highs, lows, fronts, troughs and isobar spacing. This explains where changing wind, rising air and organised cloud or precipitation are likely.
- Decode hazards and vertical limits. For every relevant SIGWX area, establish the hazard type, intensity, base and top. Compare those levels with the planned climb, cruise and descent profile.
- Compare adjacent forecast times. Consecutive charts reveal the direction and speed of a front or hazard area. A single snapshot cannot show when conditions will reach the route.
- Cross-check observations and advisories. Prognostic charts are strategic forecasts, not substitutes for aerodrome reports, terminal forecasts, SIGMETs, radar information or an official weather briefing.
How do you interpret a pressure prognosis chart?
Read a pressure chart from the large-scale pressure pattern down to individual fronts and troughs.
- Highs and lows: An
Hmarks a high-pressure centre and anLa low-pressure centre. In the Northern Hemisphere, broad airflow is clockwise around highs and anticlockwise around lows; the directions reverse in the Southern Hemisphere. - Isobars: These join places with equal mean sea-level pressure, normally labelled in hectopascals. Closely packed isobars indicate a stronger pressure gradient and usually stronger wind, but surface friction and terrain alter the actual wind.
- Cold fronts: Triangles point towards the forecast direction of movement. Expect a relatively narrow zone of changing wind, cloud and possible convective weather, although the exact conditions depend on the air masses involved.
- Warm fronts: Semicircles face the movement direction. Their cloud and precipitation can extend well ahead of the surface position.
- Occluded and stationary fronts: An occlusion combines triangles and semicircles on the same side; a stationary front places them on opposite sides. Slow movement does not mean weak weather.
- Troughs: A trough is an elongated area of relatively low pressure. It can organise cloud, showers or thunderstorms even when no conventional front is drawn.
A pressure chart indicates the synoptic setup, not the exact ceiling or visibility at an airport. Our explanation of fronts, isobars and forecast-chart symbols covers those conventions in more detail.
What do common SIGWX symbols and labels mean?
SIGWX charts use outlined hazard areas, abbreviated descriptions and flight-level figures to compress three-dimensional forecasts onto one page.
| Marking | How to read it | Common trap |
|---|---|---|
| Outlined or scalloped area | A forecast hazard region whose meaning is identified by a symbol, label or keyed number | Treating the boundary as an exact edge |
CB | Cumulonimbus activity; qualifiers may describe coverage, organisation or concealment | Ignoring the turbulence, icing, lightning and hail associated with convection |
ISOL, OCNL, FRQ | Relative spatial coverage of the phenomenon, as defined for that product | Reading them as duration rather than coverage |
EMBD, OBSC, SQL | Embedded, obscured or squall-line convection | Assuming embedded cells will be visually identifiable |
| Top and base figures | Vertical limits, commonly expressed as flight levels; the top is usually printed above the base | Reading flight levels as height above the ground |
| Jet axis with arrow and barbs | Jet-stream direction, wind speed and usually the flight level of the core | Assuming the core level is the only altitude affected by strong wind or turbulence |
| Tropopause figure | Tropopause height as a flight level, sometimes with local high or low markers | Mistaking it for temperature or wind speed |
| Turbulence area | A numbered or labelled region with intensity and vertical limits | Assuming an unmarked region is guaranteed smooth |
Flight levels are pressure altitudes referenced to standard pressure, not literal feet above mean sea level. For example, FL180 represents the pressure surface nominally associated with 18,000 feet under standard conditions.
Turbulence notation and vertical limits need particular care; our guide to understanding turbulence areas and flight levels explains how to judge whether a route intersects them. Standalone icing is more commonly presented on low-level products, so use the dedicated explanation of icing severity, bases, tops and freezing levels when that is the main concern.
Can a prognostic chart be used as the actual weather?
No. A prognostic chart is a forecast for a broad area and stated time, whereas actual weather must be established from observations and updated operational information.
A mistake we see constantly is comparing a chart valid several hours later with the weather visible at the airport and deciding that one must be wrong. First align the UTC date and time, then decode the latest METAR observations for wind, visibility, cloud, temperature and pressure. A METAR is only a point observation, so it still does not describe the whole route.
In a flight simulator, also match the simulator date, UTC time and weather source to the chart. Live weather, a preset or a downloaded historical scenario may come from a different forecast cycle, and the simulator may smooth fronts or hazard boundaries differently.
What mistakes cause most SIGWX and pressure-chart misreads?
- Using the issue time instead of the valid time: Always plan against the validity stated on the chart.
- Ignoring the chart’s altitude band: A hazard can exist above or below the levels covered by the product.
- Treating boundaries as precise: Forecast lines represent transition zones with uncertainty, not walls in the atmosphere.
- Assuming blank space means no hazard: A chart depicts only specified phenomena meeting its inclusion criteria.
- Using isobar spacing as an exact wind forecast: It indicates pressure gradient; use the appropriate wind forecast for direction and speed at altitude.
- Reading only one forecast time: Compare successive charts to understand system movement and whether the route is improving or deteriorating.
- Skipping the legend: Provider-specific symbols, units and conventions override assumptions learned from another chart series.