Weather charts & route reading

Which time and height range does this map describe?

Identify the product before decoding a coloured region or numerical label.

Analysis, prognosis and coverage

An analysis depicts an assessed atmospheric state at its analysis/valid time, assembled from observations and modelling; a prognosis is a forecast for a later valid time. Issue time, model origin time, forecast lead time and valid time are different. A chart based on 0000Z and labelled T+24 is valid at 0000Z the following day, not throughout the next 24 hours. Read day/month where a flight crosses midnight.

A SIGWX chart depicts specified significant-weather features over a stated area and vertical domain. Low-level products usually cover the surface to FL100, extended to FL150 or higher in some mountainous/regional products; use the actual legend. Medium/high legacy WAFC products and the newer multi-time products have different ranges. Read coordinates/projection, units, vertical reference, key, lower/upper domain and any masked area before using a symbol. XXX in a vertical label normally indicates a limit beyond the chart domain, not zero or unlimited hazard.

In 2025 the WAFC products changed: legacy T+24 medium/high charts remain during transition, while multi-time SIGWX spans FL100–FL600 with forecasts T+6 to T+48 at three-hour intervals. Current legacy medium charts use FL100–FL450 and high charts FL250–FL600; the consolidated EU rule text checked in October 2026 still specifies FL250–FL630 for the high-level service, while current WAFC chart legends use FL250–FL600. Older reference images may also show FL630. The January 2026 ECQB update explicitly states that Meteorology examination content follows the T+24 format introduced early in2025. Study that representation first; use the older examples only for the labelled historical comparison. Identify the chart edition and its own legend rather than applying an old limit to every product.

Does a boundary mean the same thing for CB and turbulence?

Use symbols and labels together; the shape or colour alone is insufficient.

Read significant-weather features

On a SIGWX chart, distinguish scalloped CB boundaries, turbulence regions and their keys/callouts, icing regions, jet axes, tropopause information and special phenomena such as tropical cyclones or volcanic eruptions. Labelled tops and bases are flight levels unless the legend says otherwise; a top above a base describes a layer, not the aircraft’s guaranteed encounter. A CB forecast implies the associated thunderstorm, hail, icing and turbulence hazards, so their absence as separate symbols inside a CB area is not an all-clear.

In the current WAFC style, CB areas use OCNL or FRQ coverage labels; EMBD is no longer a separate WAFC CB category after the January 2025 change. That does not remove EMBD from SIGMET terminology or mean embedded convection has disappeared. Current PNG products can use coloured icing regions and tropopause contours; older/provider-rendered displays can show different monochrome lines, callouts or point tropopause values. Read the supplied key instead of identifying a phenomenon by colour alone.

A thick jet axis shows flow along the core; arrow direction and the map grid determine direction. Speed pennants/full/half feathers represent 50/10/5 kt; core FL and any additional vertical extent must be read separately. At maxima of at least 120 kt, labelled upper/lower limits can delimit the layer with wind at least 80 kt; they are not turbulence boundaries. Change bars can indicate a 20 kt change or a 3,000 ft core-level change. A strong jet increases the need to inspect shear/turbulence guidance, but is not itself a universal CAT-area symbol.

Low-level weather and special warnings

A low-level SIGWX/area product can depict fronts, significant cloud layers and types, reduced visibility and the responsible weather, mountain obscuration, freezing-level height, icing, turbulence, mountain waves and strong surface wind. Coverage, thresholds, height reference and whether features are observed or forecast are product-specific. Relate a cloud-base forecast to terrain elevation; a base above sea level cannot be read as clearance above every ridge.

Volcanic-eruption and tropical-cyclone markers on a broad chart are prompts to read the relevant advisory and SIGMET, including forecast extent/levels and time. A marker does not delineate the complete ash or wind hazard footprint. Likewise, plotted icing/turbulence areas indicate forecast exposure at their stated levels, not a guarantee of severity in every aircraft or a complete map of unforecast small-scale hazards.

An archived medium/high comparison

The first reproduced chart is a WAFC London medium-level example valid 1200 UTC 21 October 2023, with domain FL100–450. Its CAT key entry 2 lists top FL400 and base FL180: a FL350 crossing of the corresponding area is within that vertical interval, while FL410 is above this particular layer. A callout 320/XXX for CB gives a top at FL320 and a base below the chart’s lower domain, not a surface base of zero.

The second, high-level example is valid 1200 UTC 16 July 2024 and has its own FL250–630 legend. A jet-core label such as FL500 gives the pressure-altitude level of that core, not a speed of 500 kt. Boxed tropopause values such as 550 give the tropopause flight level in this older point-display style. Use the jet arrow and map grid for flow direction.

These are archived styles: the isolated/embedded CB labels and point tropopause display must not be assumed to be the current WAFC PNG style. The January 2025 guide and actual product legend govern the newer representation. The image is an authentic decoding exercise with its original dates, not a forecast for today.

Which features changed while the height and time checks remain essential?

Start with the WAFC London guide examples for the format introduced on 23 January 2025. Their demonstration forecast is dated 19 September 2024; the images are archived training examples, not weather for a present flight.

Current-format high-level interpretation

The Area C high-level example states FL250–FL600, with a T+24 forecast valid at 0600 UTC 19 September 2024 based on the 0600 UTC 18 September run. The red scalloped cloud areas use OCNL CB labels. A label 340/XXX near Italy gives a CB top at FL340 with base below this chart domain, not a surface base at 0 ft. Thin blue dashed contours carry tropopause flight levels; they are not cloud boundaries or isobars.

On the northern jet, FL330 labels the core level and 230/400 gives the lower/upper limits of the layer with wind at least 80 kt around the qualifying maximum. The arrow shows flow travel; these two limits are not 230 kt and 400 kt. Read the grey dashed turbulence regions with the separate numbered key. Distinguish the jet axis, turbulence layer and tropopause contours even where they lie close together.

Current-format medium-level interpretation

The EUR medium-level example states FL100–FL450. Purple shaded areas carry icing callouts; red scallops still indicate CB, and grey dashed shading with a numbered key indicates non-convective turbulence. In the bottom key, turbulence area 2 extends from FL160 to FL400, so a route crossing that area at FL180 intersects its forecast layer. The aircraft must also be horizontally inside the area at the relevant time; the key alone does not place the whole map in turbulence.

An icing callout 260/120 gives top FL260 and base FL120. A callout 220/XXX gives a top at FL220 with the base below the FL100 domain. Read the actual symbol and its top/base pair rather than transferring the colour meaning from another provider. Convection carries its associated icing/turbulence/hail hazards; an independent icing or turbulence area is not required to repeat every hazard within a CB. The older charts later in this chapter are labelled historical comparisons and do not replace these new-style conventions for ECQB 2026.

How do the Canary cloud labels relate to a route altitude?

Use the archived AEMET example below; its dates identify a teaching document, not current flight information.

Read the Canary Islands chart

The reproduced image is the Canary Islands SFC/FL150 chart with a central valid time of 0000 UTC on 28 May 2024. Its stated window is the three hours before and after that time. Read its own legend: heights are in hundreds of feet above mean sea level, not automatically above each island or airport. The chart is reproduced with attribution from AEMET GuíaMET, updated 17 October 2025; do not use this archived forecast for a present flight.

In the broad blue region, BKN/LCA BKN CU SC describes the cloud amount/local qualification and cloud types. The displayed top range 030–040 means 3,000–4,000 ft AMSL; the base range 015–025 below the dividing line means 1,500–2,500 ft AMSL. These are forecast ranges within the region, not exact heights at every point. A low route or terrain between those bounds can be affected by cloud even if the aircraft starts below it over the sea.

The boxed 0°:XXX indicates that the freezing level lies above the chart’s vertical domain in this example; XXX is not zero height or a guarantee of no freezing anywhere. Cross-check the source legend and any cloud/icing/turbulence callouts. For a proposed crossing, combine location, terrain elevation, forecast base/top ranges, valid time and the actual reports rather than assigning every island the same clearance.

Which leg intersects the forecast hazard at FL350?

Read a route as a sequence of positions, levels and expected crossing times.

Horizontal overlap is only the first test

First plot the departure, route, destination, alternates and expected positions at relevant valid times. Check whether each leg enters a feature’s horizontal boundary. Then compare the planned flight level, climb/descent profile and vertical forecast limits. An area of moderate turbulence between FL320 and FL380 is relevant to a FL350 crossing, while a FL410 cruise is above that prescribed layer at the same point. It still needs separate assessment of other hazards, climb/descent and uncertainty.

A CB area with tops FL420 is not safely overflown at FL410 merely because some cells may be lower than the regional maximum. Avoidance requires the relevant operational criteria, current detection and development margins. A low-level layer can also affect departure or arrival even if cruise is above it. A route that misses a drawn polygon by a small amount is not guaranteed clear: forecast resolution, evolution and positioning errors matter.

Use the chart nearest the route’s crossing time, compare adjacent forecast times, and check SIGMET/AIRMET, METAR/TAF, radar/satellite and air-reports as appropriate. The traditional ±3-hour usability guidance for fixed-time WAFC charts does not make rapidly evolving weather stationary or override newer warnings. If no adequate coverage exists, obtain the appropriate briefing rather than treating the blank area as a favourable forecast.

Where are the warm sector, frontal side and rear?

Read the isobars and fronts together rather than inferring the whole weather from an H or L.

Centres, elongated features and the saddle

Surface analysis and forecast charts normally reduce pressure to mean sea level and join equal pressure with isobars. A closed low has lower pressure towards its centre; a high has higher pressure towards its centre. A trough is an elongated relatively low-pressure axis, and a ridge an elongated high-pressure extension. A col is a saddle between two highs and two lows, often with a weak horizontal pressure gradient near its centre. A trough need not carry a front, and a ridge does not guarantee cloud-free air.

In an ideal Northern Hemisphere mid-latitude frontal low, identify the warm front extending ahead of the warm sector, the trailing cold front, and any occluded portion near the centre. The warm sector lies between the surface warm and cold fronts. The frontal/ahead side often has rising layered cloud and precipitation; the rear cold air can have showers, clear intervals and strong gusts depending on stability and surface heating. Do not assign identical weather to every low: moisture, development stage, season, terrain and upper support alter the pattern.

Warm-front semicircles and cold-front triangles lie on the side towards which the boundary advances. An occluded front uses alternating symbols on the same side; a stationary front uses alternating symbols on opposite sides. Compare successive analyses/prognoses for motion and development. A surface front is a boundary at the surface, while the associated cloud and wind change can extend ahead and above it.

Can isobar spacing alone give an exact runway wind?

Pressure geometry gives a useful regional estimate with explicit assumptions.

Direction and speed

Away from the equator and strong curvature/friction, geostrophic wind is approximately parallel to isobars, with low pressure to its left in the Northern Hemisphere and right in the Southern Hemisphere. Surface friction reduces speed and turns the flow across isobars towards lower pressure. Thus Northern Hemisphere flow around a low is anticlockwise with inward crossing near the surface; the Southern Hemisphere sense reverses. Local terrain, convection and coastal effects can dominate a runway observation.

Tighter spacing means a larger pressure gradient and usually stronger wind for the same latitude and chart scale. To estimate speed numerically, use the chart’s geostrophic wind scale with its stated isobar interval, true map distance and latitude, then apply an appropriate surface/curvature allowance. If a scale or sufficient distance/latitude information is absent, a precise speed cannot be read from spacing alone.

As a worked exercise at 50°N, a 4 hPa difference across 100 km gives a gradient of 0.004 Pa/m. Using sea-level density 1.225 kg/m³ and f = 2Ω sin50° ≈ 1.12 × 10⁻⁴ s⁻¹ gives Vg ≈ 29 m/s, about 57 kt. A prescribed land friction factor 0.5 would suggest about 28 kt with inward cross-isobar flow, not a universal surface prediction. Pressure-height reduction, stability, roughness and curvature limit that approximation. In tightly curved flow use the gradient-wind context and actual observations.

What changes along a 500 hPa chart?

A constant-pressure map is a surface of equal pressure with variable height and weather.

Four different line families

A constant-pressure chart depicts conditions on a specified pressure surface, such as 500 hPa, rather than on a flat surface of constant geometric altitude. That pressure surface is higher over a warmer/thicker underlying column and lower over a colder/thinner one, subject to the full pressure and temperature structure. Its height varies across the map; 500 hPa is only approximately near FL180 in a standard atmosphere.

An isohypse joins equal geopotential height on the pressure surface, often labelled in geopotential metres or decametres. A contour labelled 552 dam corresponds to 5,520 geopotential metres, not a pressure of 552 hPa. An isotherm joins equal temperature, and an isotach joins equal wind speed. An isobar joins equal pressure on an appropriate reference surface; do not use the terms interchangeably.

Upper flow approximately follows height contours under geostrophic conditions, with lower heights to the left in the Northern Hemisphere and to the right in the Southern Hemisphere. Closer height contours at a given latitude generally indicate stronger flow; isotachs directly delineate wind-speed regions and a jet maximum. Troughs and ridges in the height field, temperature gradients and wind maxima combine to describe the upper pattern. Direction is referenced to local geographic north/grid, not merely the top of the page on a polar projection.

What wind should you use halfway between two forecast points?

Read units, level and reference before averaging the data.

Forecast wind/temperature charts

Upper wind and temperature forecasts can be plotted at selected flight levels or pressure levels, as wind barbs/arrows and numerical temperatures. A wind barb’s shaft points towards the direction from which the wind comes; a pennant is 50 kt, a full feather 10 kt and a half feather 5 kt. This differs from a jet-axis arrow, which points along flow travel. Read the sign convention: some coded/plotted products omit a minus sign for specified temperature contexts or use special compact coding, so the legend is essential.

For a point between grid points or a route between levels/times, interpolate only within the available product’s domain and appropriate resolution. Scalar temperature can be linearly interpolated for a simple examination exercise. Wind should be interpolated as vector components, especially near north or when directions differ greatly. Halfway between equal winds from 350° and 010° is near 000°, not 180°. The speed of the vector mean is slightly less than the individual speeds when they diverge.

For ISA deviation, calculate ISA temperature at the specified pressure altitude/FL and subtract it from OAT: deviation = OAT − TISA. At FL100, using 2 °C per 1,000 ft for a rounded examination approximation, TISA is −5 °C. An OAT of −10 °C is ISA −5, not ISA +5. Use the stated lapse-rate convention when precision is required; exact ISA uses 6.5 °C/km below the tropopause.

A route average has a weighting

A simple midpoint/average exercise may use equally spaced points, but an operational route average should account for the applicable leg lengths or times and changing level/time. For interpolation at one point, average wind vectors before deriving direction and speed. For a route-wide along-track summary, first resolve each leg’s assigned wind against that leg’s track, then apply the specified distance or time weights. One global mean wind vector cannot preserve each leg’s headwind when the route direction changes; averaging directions or unweighted components can mislead. Grid interpolation smooths fields and cannot locate every small-scale gust, wave or cell.

Example: equal vector weights for 350°/40 kt and 010°/40 kt cancel their east–west components and retain a 39.4 kt wind from north. If corresponding OATs are −8 and −12 °C at FL100, the midpoint OAT is −10 °C. With the rounded −5 °C ISA reference, the deviation is −5 °C. These are prescribed interpolation results, not a new forecast or a guaranteed aircraft performance input.

Can you apply the ideas?

Four original questions. Decide from the facts, then review the explanation.