Chart family and validity
SIGWX and wind-temperature charts are planning products. Before reading a symbol, check the header, issuing centre, area, validity time and vertical range. A chart is useful only when it matches the route and the period being planned.
International production
World area forecast centres prepare international forecast products for flight planning. WAFS distribution gives access to high-level significant weather and wind-temperature information.
Use the chart as a forecast
A chart describes expected significant features, not a complete observation. Confirm current conditions with reports and warnings before flight.
Symbols and vertical limits
SIGWX symbols identify hazards such as CB, turbulence, icing, jets, tropopause and freezing level. The printed vertical limits and intensity are as important as the symbol itself.
Jet and CAT
A jet axis may indicate strong wind and associated clear-air turbulence. Read maximum wind, level and CAT area together. The jet is not a guarantee of turbulence at every point.
Cloud and freezing
CB areas, freezing level and tropopause information must be related to planned level and terrain. The chart helps the crew choose where a route crosses a hazard vertically.
Forms 215 and 415
High-level SIGWX forms cover the upper flight levels. They show significant weather relevant to high-level operations and must be read with their legend, validity and height conventions.
What to extract
Identify CB and thunderstorm areas, turbulence and icing intensity, jet axes, tropopause and freezing information, volcanic ash and tropical cyclones where plotted.
Planning value
The high-level chart supports route and level selection. It does not replace tactical weather avoidance or current SIGMET.
Forms 214 and 414
Low-level significant weather charts use a different vertical range and need the same discipline: read title, validity, symbols and limits. Their cloud, weather, icing and turbulence information is especially relevant to climb, descent and lower-level routes.
Low-level reading
A low-level chart should be used with terrain and aerodrome forecasts. A hazard close to high ground has a different clearance and escape implication from the same hazard over sea.
Chart pairing
Use the low-level and high-level charts together for a full vertical flight profile. Do not assume the absence of a feature on one proves its absence at the other level.
Wind-temperature interpolation
Wind-temperature charts plot forecast wind and temperature by location and level. Interpolate only between nearby values on the same air mass and chart period. Rapid changes near fronts and jets require extra caution.
Groundspeed and ISA deviation
Mean wind direction and speed affect groundspeed. Forecast temperature compared with ISA affects aircraft performance and can help identify unusual air-mass conditions.
Route segmentation
Break the route where wind or temperature changes rapidly. A single average for a long route can hide a headwind sector or a jet crossing.
National comparison and chart brief
Joshi's national SIGWX and Met T3/T4 material supplements the international chart family. The teaching point is the same: use the correct product, height range and validity for the flight.
A disciplined brief
Read chart header, area, valid time and vertical range. Mark the route. Extract significant weather, wind and temperature by segment. Compare with warnings, METARs, TAFs and satellite or radar evidence.
Decision not decoration
Charts are not pictures to glance at. Every highlighted feature should produce a route, level, fuel, timing or alternate consequence.
Chart values to extract
High-level SIGWX Forms 215 and 415 cover FL250 to FL630. They show CB, turbulence, icing, jet axes, tropopause, freezing level, volcanic ash and tropical cyclone information as applicable. Forms 214 and 414 cover the lower range and are read with terrain, departure and approach in mind.
Route interpolation
On the wind-temperature chart, interpolate along each route segment and at the chosen level. Do not draw one average through a frontal zone or jet. Wind direction and speed determine groundspeed; forecast temperature compared with ISA gives ISA deviation. WAFC products are distributed through WAFS and SADIS. Joshi's national SIGWX and Met T3/T4 products supplement the international products, but their title, issue time and height references must be checked before comparison.
Symbol discipline
A jet symbol is read with its axis, maximum wind and level. A CAT area needs its intensity and upper and lower limits. CB is read with cloud base and top where supplied. The tropopause label gives its height, while the freezing-level line gives the approximate zero-degree level. A symbol without its limits is an incomplete briefing.
Pressure or height changes rapidly near a frontal zone, mountain range or jet. Therefore a route interpolation should be split at those boundaries. A tailwind on the first half of a route does not offset a strong headwind or turbulence diversion on the second half when calculating the operational fuel case.
Source-based route example
For a route crossing a jet, mark the jet axis and maximum-wind level from the wind chart, then draw the CAT area and its limits from SIGWX. If the selected level lies in the CAT layer, compare an adjacent level and calculate the revised wind effect. If a frontal boundary lies between plotted stations, do not interpolate through it as though wind and temperature changed smoothly.
Wind-temperature worked method
At each route waypoint, read the nearest plotted wind direction, wind speed and temperature for the selected level. Interpolate between two nearby plotted values only when the air mass is continuous. Convert the wind to headwind or tailwind on the track, then calculate groundspeed by segment. Compare forecast temperature with ISA for the pressure altitude to obtain ISA deviation. Repeat at an alternative level before selecting the fuel and turbulence optimum.
Low-level and high-level charts are complementary. A climb can be affected by low-level icing while the cruise remains clear, and a jet CAT area can be above a low-level route. The chart header, range and validity decide which conclusion is valid.
Before using a plotted wind
Confirm whether direction is true, whether the value is forecast for a pressure surface or a flight level, and whether the chart time matches the route time. A charted temperature is an air temperature at the stated level, not an outside-air-temperature observation from the aircraft. These details determine whether the wind can be used for groundspeed and performance planning.
Pressure-height caveat: a wind chart may be plotted for a pressure surface while the operational flight plan uses a flight level. Temperature and pressure variation change the geometric height of that surface. Read the chart convention before comparing terrain clearance, tropopause height or aircraft altitude with the plotted wind.