Ch 13 Quiz: Weather Charts — Surface, Upper Wind & Prog Charts

Synoptic chart (surface analysis): drawn from simultaneous weather observations at the same time (synoptic time: 00, 06, 12, 18 UTC). Shows: isobars, fronts, high/low pressure centres, and station model data. The foundation of weather analysis.

Isobars: lines connecting points of equal pressure (reduced to MSL). Drawn at 2, 4, or 8 hPa intervals. Closely spaced isobars → steep pressure gradient → strong wind. Widely spaced → gentle gradient → light wind.

Trough: elongated area of relatively low pressure. Isobars are V-shaped pointing toward the high pressure side. Troughs are associated with convergence, cloud, and precipitation. Fronts are a special type of trough (temperature discontinuity).

SIGWX chart area symbols show the VERTICAL EXTENT of the hazard: lower altitude (above line)/upper altitude (below line). FL050–FL250 = phenomenon extends from FL50 to FL250. ICAO SIGWX charts are issued for FL100–FL450 (high level) and FL100–FL250 (medium level).

High-level SIGWX (FL250–FL630): issued by World Area Forecast Centres (WAFC London and Washington). Each chart is a snapshot valid at a specific time (T+24h, T+36h). Covers: CB, jet streams, tropopause height, turbulence, icing, and tropical cyclones.

Thickness: difference in height between two pressure levels (usually 500–1000 hPa). Thickness ∝ mean temperature of the layer (warm air expands → thick layer). The 5,400 m (540 dam) 1000–500 hPa thickness is often used as a rain/snow discriminator.

Upper-level charts (e.g. 500 hPa, 300 hPa) use GEOPOTENTIAL HEIGHT CONTOURS (like isobars). Wind blows parallel to these contours (geostrophic approximation). Closely spaced contours → strong wind. A trough = dip toward equator; ridge = bulge toward pole.

300 hPa (~30,000 ft / FL300): close to the jet stream level. The 300 hPa chart shows wind isotachs (speed contours), jet stream axes, and areas of maximum wind helpful for route planning. Also used to locate CAT near strong wind speed gradients.

Weather radar charts: composite reflectivity maps from networks of ground-based weather radars. Show precipitation intensity (light/moderate/heavy/extreme), storm cells, and movement. Used in dispatch and approach briefing. Do not show CAT (no precipitation).

On international charts, a scalloped (wave-edged) boundary encloses areas of IMC/IFR conditions (cloud ceiling <1000 ft and/or visibility <3 sm). Smooth boundary = marginal VFR (MVFR). This helps VFR flight planning.

IR satellite imagery: measures outgoing long-wave radiation (temperature). Cold high cloud tops = bright white (highest, coldest). Warm surface/low cloud = grey/dark. Allows detection of Cb tops (very cold/bright), fronts, and cloud distribution at night.

Visible (VIS) satellite imagery: reflected solar radiation. Very useful for: distinguishing thick Cb from thin cirrus (texture), seeing fog/stratus (textured vs. smooth), and surface features. Only available in daylight. IR available 24 hours.

PROG (prognostic) chart: a forecast of the atmospheric state at a specified future time, showing expected isobars/fronts (surface) or contours/wind (upper). Issued at T+12h, +24h, +48h, +72h. Used for flight planning beyond the TAF window.

Tropopause height is shown on SIGWX (high-level) charts (in FL, e.g. FL360). Lower tropopause (polar regions, ~FL300) → greater chance of encountering Cb tops at cruise; higher tropopause (tropics, ~FL500–550) → deeper convection possible. Important for turbulence and clearance.

Two WAFCs: WAFC London (UK Met Office) and WAFC Washington (NOAA/NWS). Together they provide global coverage. WAFC London products cover Europe, Africa, Asia, and Australasia (including India). Products available via SADIS (Satellite Distribution System) and internet.

H (high/anticyclone): closed circulation of clockwise-rotating (NH) sinking air. Clear skies, light winds. L (low/cyclone): anti-clockwise rotating (NH) rising air. Unsettled weather, fronts, wind, cloud, precipitation. Centres are labelled with central pressure value.

Col: the saddle point of pressure between two highs and two lows (arranged alternately). Light, variable winds (weak pressure gradient). In summer: strong surface heating with no wind → convective storms. In winter: fog and radiation inversions → persistent IMC.

500 hPa chart (~18,000 ft / 5.5 km): close to the ‘steering level’ for mid-latitude systems. Troughs at 500 hPa guide the movement of surface lows and fronts. Widely used for medium-range forecasting. Height contours, temperature (isotherms), and vorticity shown.

Blocking high: a large, persistent (days to weeks) anticyclone, often at 50–70°N, that diverts the jet stream and blocks eastward-moving lows. Causes prolonged extreme weather: heatwaves (downstream), cold spells (upstream), floods (near blocked system).

Rex block: upper-level HIGH directly over (or slightly poleward of) an upper-level LOW. The pair stalls the flow. The high prevents normal W–E progression. Results in prolonged rainfall (southern low) and dry/warm conditions (northern high) — high-impact weather pattern.

WAFC SIGWX products: freely available via ICAO SADIS (broadcast via Inmarsat) and WIFS (internet). Any aerodrome meteorological office or flight operations centre can access global high-level SIGWX, upper wind/temperature, and BUFR grids.

AIREP: Air Report. ICAO standard format for in-flight weather observations from commercial aircraft. Transmitted via HF radio to MWOs. Contains: position, time, FL, OAT, wind, turbulence, icing, cloud. Now largely superseded by ACARS automated reports but still used on oceanic routes.

Medium-level SIGWX: FL100–FL250. Covers: CB/TS, MOD/SEV turbulence, MOD/SEV icing, mountain wave. Designed for aircraft operating below the high-level chart’s coverage. Issued by WAFCs with the same BUFR digital products now replacing traditional charts.

Vertical cross-section: a slice through the atmosphere along a route or latitude/longitude line. Shows: cloud, icing zones, freezing level, tropopause height, jet stream, turbulence. Extremely useful for pre-flight planning at different altitudes along a route.

250 hPa chart (~34,000 ft / 10.4 km): near the jet stream level. Contours = geopotential height of the 250 hPa surface. Isotachs (wind speed contours) also shown. Used to locate jet stream axes, jet streaks, and areas of upper-level divergence/convergence.

NWP models: GFS (Global Forecast System, NOAA), NAM (North American Mesoscale), ECMWF (European Centre), UK Met Office UM, India IMD Global model. They ingest global observations, run complex atmospheric equations on supercomputers, and produce grid-point forecasts up to 10+ days ahead.

Enhanced IR: IR image with colour enhancement table so that very cold cloud tops (Cb, MCS) stand out in distinctive bright colours (often yellow/orange/red for <−50°C). Essential for convection monitoring at night when visible imagery is unavailable.

CB EMBD (embedded Cumulonimbus): one of the most dangerous SIGWX annotations. Cb hidden within layer cloud. Pilots entering EMBD areas cannot see or avoid the storm without onboard weather radar. Enclosed by a zigzag hatched line on SIGWX charts.

Analysis time: the ‘T=0’ time, representing when all the observations shown on the chart were taken (synoptic observation time: 00, 06, 12, or 18 UTC). Distinct from issuance time (when the chart was drawn/released) and forecast valid time (for prog charts).

BUFR: WMO standard for encoding meteorological data in binary format. Used for SIGWX, WAFS products, gridded data exchange. Replaces paper/fax charts with machine-readable data that flight planning systems can directly ingest. ICAO has mandated BUFR/XML for OPMET since 2023/2025.