earth science

SIGMET

SIGMET is a earth science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand SIGMET rather than just read about it. In short: SIGMET, or Significant Meteorological Information (AIM 7-1-6), is a severe weather advisory that contains meteorological information concerning the safety of all aircraft. Compared to AIRMETs, SIGMETs cover more severe weather.

Key takeaways

SIGMET belongs to earth science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect SIGMET to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of SIGMET from memory before moving on to harder problems.

Reference excerpt

SIGMET, or Significant Meteorological Information (AIM 7-1-6), is a severe weather advisory that contains meteorological information concerning the safety of all aircraft. Compared to AIRMETs, SIGMETs cover more severe weather. Today, according to the advancement of technology in civil aviation, the SIGMET is sent as IWXXM model.

Types There are three main types of internationally recognized SIGMETs per ICAO:

Volcanic ash (VA or WA SIGMET) Tropical Cyclone (TC or WC SIGMET) Other En-route weather phenomenon (WS SIGMET), which may consist of Thunderstorm types Turbulences types Mountain waves Icing/Sleet/Hail Dust or sand storms Radioactive Cloud This information is usually broadcast on the ATIS at ATC facilities, as well as over VOLMET stations. They are assigned an alphabetic designator from N through Y (excluding S and T). SIGMETs are issued as needed, and are valid up to four hours. SIGMETS for hurricanes and volcanic ash outside the CONUS are valid up to six hours.

Convective SIGMETs

For airmen in the U.S., there is an additional category of SIGMET known as a convective SIGMET. These are issued for convection over the coterminous U.S. on a scheduled basis, hourly at 55 minutes past the hour. The Convective SIGMETs are valid for 2 hours or until superseded by the next SIGMET. If it is determined that the conditions do not meet the Convective SIGMET criteria within a given region, a CONVECTIVE SIGMET... NONE is issued. Additionally, a 2 to 6-hour outlook is attached at the end of each regional bulletin. There are three types of convective SIGMETs:

When shown in a graphical depiction, the convective SIGMET polygon is a "snapshot" that delineates the region of thunderstorm at the issuance time of 55 past each hour. There are also a few special issuance convective SIGMETs to cover extreme weather more common in the U.S., including:

Tornadoes Hail greater than or equal to 3/4 inches in diameter Wind gusts greater than or equal to 50 knots Indications of rapidly changing conditions not sufficiently described in existing convective SIGMETs.

Structure SIGMETs are internationally used by ICAO and WMOs using standardized abbreviations

and are split into three lines:

Header Summary Main Body In sum, a standardized SIGMET will have the following structure: TTAAii CCCC YYGGgg [BBB] CCCC SIGMET [n][n]n VALID YYGGgg/YYGGgg CCCC- CCCC <FIR/CTA Name> FIR <Phenomenon> OBS/FCST [AT GGggZ] <Location> <Level> [MOV XXX xx KT/KMH] [INTSF/WKN/NC] <Forecast time and forecast position>=

Header The header line consists of the following characters TTAAii CCCC YYGGgg [CCx]

Summary The first line of the broadcast is a summary line consisting of the following characters CCCC SIGMET [n][n]n VALID YYGGgg/YYGGgg CCCC-

Notes [a] – if it a convective SIGMET, then this will read SIG[E/C/W] CONVECTIVE SIGMET ##[E/C/W]. Where E/C/W indicates whether it's over the Eastern, Central, or Western United States, and ## indicates the number of the convective SIGMET issued for that region.

Body The main body of a SIGMET can be much more variable, and consists of CCCC [FIR/CTA list] <Phenomenon> OBS/FCST [AT GGggZ] <Location> <Level> [MOV XXX xx KT/KMH] [INTSF/WKN/NC] [FCST AT <GGgg>Z <location>]= CCCC [FIR/CTA list] is again the 4-character ICAO location, followed by the affected flight or control regions. <Phenomenon> is a code describing the meteorological phenomena as follows:

If it is a convective SIGMET, the following codes may appear

OBS/FCST [AT GGggZ] indicates whether the phenomenon is observed (OBS) or forecasted (FCST), and the Zulu hour and minute that it was observed or will be forecasted. <Location> is a general description of location of the meteorological phenomenon, typically utilizing latitude and longitudinal coordinates. <Level> helps denote the altitude that the phenomenon will be occurred, and can be expressed as

[MOV XXX xx KT/KMH] if it is a moving front, the direction and rate of movement given as a compass direction (XXX, e.g. "N" or "WNW"), and the rate is given in KT (or KMH). Sometimes STNR (Stationary) may be used instead if no significant movement is expected. [INTSF/WKN/NC] denotes the change in strength over time. [FCST AT <GGgg>Z <location>] helps note where the front is expected to be at the end of the SIGMET's validity period.

SIGMET Renewal and Cancellation If when the validity period is due to expire but the phenomenon is expected to persist, a new sequence number is added to the SIGMET to renew it. If during the validity period of a SIGMET, the SIGMET is to be cancelled, the following replaces the SIGMET message

CNL SIGMET [n][n]n YYGGgg/YYGGgg

In the United States, non-convective SIGMETs have a valid period no more than 4 hours. If the phenomena continue, a new SIGMET will be reissued at least every 4 hours. If an amendment is needed, a new SIGMET will be issued using the next series number. Corrections to SIGMETs are issued as necessary, and are identified with COR.

Examples WSUS32 KKCI 071655

An en-route weather phenomenon in the U.S., issued by the Aviation Weather Center in Kansas City, MO on the 7th of August, at 16:55 UTC

SIGC CONVECTIVE SIGMET 83C

This is a convective weather pattern in the central region of the contiguous U.S. with sequence number 83C

VALID UNTIL 1855Z

That is valid until 18:55 UTC

MI IN WI IL IA LM

That covers Minnesota, Indiana, Wisconsin, Illinois, Iowa, and Lower Michigan flight regions

FROM 30E GRR-30S GIJ-30SSW BDF-10ENE IOW-50ENE DBQ-30E GRR

A description of the area of the storm, where GRR (Grand Rapids, MI airport), GIJ (Niles, MI VORTAC), BDF (Bradford, IL VORTAC), IOW (Iowa City, IA airport), and DBQ (Dubuque, IA airport) are ConTrol Areas (CTAs).

AREA TS MOV FROM 25025KT. TOPS TO FL450.

Thunderstorms moving from 250 degrees (roughly WSW) at 25 knots, covering an altitude from the cloud cover to 45,000 ft.

Illustrations

Worked examples

Example 1 — a first encounter with SIGMET

Start with the simplest possible case. Write down what SIGMET claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In earth science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to SIGMET before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about SIGMET ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of SIGMET

In research: SIGMET appears in earth science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses SIGMET in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: SIGMET is common in secondary-school and first-year university syllabi. It links to neighbouring topics Aviation meteorology, Aviation publications, Earth sciences data formats, so understanding it makes those chapters shorter.
In everyday life: Look for SIGMET outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study SIGMET in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what SIGMET means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain SIGMET out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is SIGMET in simple terms?

Why does SIGMET matter?

Because it connects several earth science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study SIGMET?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on SIGMET.