Aviation significant weather (SIGWX) charts
This is a brief note relating to what you will see on aviation significant weather (SIGWX) charts that are issued by many centres around the world, but principally looking at those issued by WAFC [World Area Forecast Centre] London (actually a unit within the Operations Centre in Exeter [Devon], having been in Bracknell until 2003, and before that, as RAFC [Regional Area Forecast Centre] London, at London/Heathrow airport until the 1980's).
( Note that the only other WAFC issuing products is called "Washington", but is in fact in Kansas! )
It is NOT intended to replace (and should not be used as) formal advice to pilots/aircrew but rather it is designed to be a quick 'aide-memoire' to interested amateurs who want to make sense of the charts.
Aviation flight-planning charts are usually issued to cover three broad 'operating levels' in the troposphere / lower stratosphere:
- low-level charts (roughly below 10000ft),
- medium / high level charts (roughly 10000ft to 45000ft) and
- high level charts (roughly above 25000ft).
In general, the lower the level, the less area a particular chart will cover; they are generally issued every 6 hours, with a fixed validity time 00, 06, 12 or 18 UTC. Bear in mind though that there will be national and military departures from these general ideas.
You will find depicted on these charts all or some of the following:
- Jetstream locations, altitude, strength, variations.
- Clear Air Turbulence (CAT) areas, height-bands, intensity.
- Tropopause levels, including 'highs' and 'lows'.
- Cloud-weather (icing/turbulence) areas, base/tops and continuity/frequency.
- Fronts, type & movement thereof.
- Tropical Storms
- Volcanic activity.
Two symbols may not be understood - they depict icing and turbulence thus:-
(Light icing/turbulence is not found on the vast majority of charts - they may be indicated on some specialist/military charts)
... bold arrow following the wind ... showing location of core of winds >=80kn ... median height of core in FL notation (warm side of jet) ... where jet core strength is >=120kn, the spread vertically above/below the jet core (+/-) in hundreds of feet that the wind equals/exceeds 80kn ... short double-parallel strokes across jet showing steps of 30kn ... conventional speed notation (i.e. triangles 50kn, long feathers 10kn, short feathers 5kn).
... dashed lines enclosing areas of similar turbulence structure (intensity/height bands) ... base/top of layer (standard FL notation) ... XXX means below base level of chart ... number in square box refers to side-panel where is found the height/intensity information. (NB: not all jets generate significant turbulence).
... spot numbers in rectangular boxes (standard FL notation) ... boxes with upward pointing arrow denote tropopause highs ('domes') ... boxes with downward pointing arrow denote tropopause lows ('funnels') ... (tropopause level is where coldest air [aloft] is found in any particular airmass ... thus the level of greatest engine efficiency).
... scalloped lines around areas of similar 'cloud' weather .. amounts / types of cloud in standard notation ... moderate / severe icing / turbulence ONLY indicated ... top / base of layer in FL notation ... note carefully: the top / base of the significant icing / turbulence layer is forecast, NOT the top / base of the clouds ... XXX means base of layer is below the lower boundary of the chart (or above the upper boundary of a low-level chart).
Specific notes regarding CB:
> Cumulonimbus (CB) imply severe icing and turbulence ... the symbols are not shown.
> ISOL CB only indicated on civil medium/high charts where embedded (EMBD) in layer cloud. Isolated CB in unstable air-masses that are not likely to be embedded are not shown on WAFC charts. However, they may be indicated on medium/high-level military charts, and also on all low-level charts, e.g. F215/415.
> OCNL/FRQ CB always shown, unless outside the vertical envelope of the chart.
... shown using standard notation AT THE SURFACE ... upper fronts and troughs are NOT indicated (on civil charts) ... occasionally expected movement is shown using an arrow and speed in knots.
... location and name (or sequential numerical identifier in some basins) of Tropical Storms (with of course the associated cloud-weather structure).
... the location of active volcanoes (sufficient to pose a hazard to in-flight aircraft from ingestion of ash/debris etc.) is shown, with its name and location offset ... users should refer to latest SIGMET's for further information.