Aviation Topic of the Week
By Michael Oxner, April 18, 2004


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This week's topic:
SIGMETs

The last couple of weeks were spent dealing with NOTAMs, an important piece of information used while planning flights, and important during flight, too. This week, we'll tackle something else of primary importance to flight, the SIGMET.

Introduction
When SIGMETs are Issued
Example

Introduction

SIGMETs are not so detailed in their basic description or definition, but can be detailed in their content. A SIGMET is the acronym for SIGnificant METeorological information. The primary reference for this topic is the AIP Canada, the dreaded MET section, paragraph 3.18. The purpose of a SIGMET is to provide pilots (and ATC as well as dispatchers and anyone else interested) with short-term weather information on significant weather related phenomena which may affect flight safety. International agreements have been established for standardization of which phenomena warrant SIGMET production, and this list is generally related to the more serious weather hazards.

SIGMETs are distributed via teletype to Air Traffic Service facilities, including ATC and FSS units across the country. Upon receipt, ATS personnel will broadcast the SIGMETs to any aircraft they are in communication with who are known to be operating in, or who will be operating in, the affected area. If a SIGMET affects the area around an airport served with an ATIS, the SIGMET may be recorded for broadcast there, as well. SIGMETs are generally valid for a period of 4 hours, but their period of validity is specified in the text.

To identify a SIGMET, they are coded with a letter and a number. Each letter is given for a specific forecast, typically related to one specific weather system or type of information, and they are numbered for continuity. As one SIGMET regarding a certain condition expires, a new one is often issued to replace it to update the conditions. In such a case, the same letter will be used, but the number will be incremented. See the next section for an example of SIGMET issuing.

When SIGMETs are Issued

The AIP mentions a number of conditions that generally warrant the issuance of a SIGMET. Their list includes:
If any of these conditions exist, or area forecast to exist, you can bet a SIGMET will be issued. Often, several SIGMETs are issued surrounding a certain weather system. For example, one might be issued for severe icing and another issued for severe turbulence if the areas in which each condition is forecast to exist in a slightly different area of a given storm system. This is why it's important to review all SIGMETs issued for your route of flight.

As mentioned above, SIGMETs are issued with a letter-number combination. This generally begins with 1 in the number area, and as each successive SIGMET issued replaces an existing one, the number is incremented. For example, SIGMET A1 is issued relating to a developing line of thunderstorms and is given a four hour period of validity with expectations of the condition continuing. About four hours after the initial SIGMET is issued, SIGMET A2 will be issued if the conditions are persisting. This will automatically cancel A1, and update the area as weather systems normally move over time. SIGMET A3 will replace A2 and so on, until the last SIGMET in the A series eventually tells us that the conditions no longer exist, such as often occurs after nightfall when the system loses the daytime heating that often fuels a thunderstorm line.

Example

The following example is taken out of the AIP in section MET 3.18. They presented it in a chart form, but I would rather present it in the form in which it would be seen from a teletype print out. I am providing this example to give you an idea of just how these things are coded. I think meteorologists are like doctors when it comes to the things they write. They don't intend to make things difficult to read. It just happens. Most of them are decipherable when you look at the context, though.

WSCN33 CWTO 171805
SIGMET A5 VALID 171805/172205 CWTO
WTN 30 NM OF LN /4622N07925W/NORTH BAY -/4458N07918W/ MUSKOKA -/4302N08109W/LONDON.
TS MAX TOPS 300 OBSD ON RADAR. LN MOVG EWD AT 20 KT. LTL CHG IN INTSTY.

Line 1 refers to the Graphic Area Forecast area 33 (GFACN33). It was issued by the Toronto Forecast Centre on the 17th day of the month at 1805z.

Line 2 identifies the SIGMET name and the sequence number. In this case, A5 is issued to update, and therefore replace, A4 for the same weather phenomenon in the same GAF area. It also gives the period of validity, from 1805 to 2205z, a for hour window.

Line 3 is the description of the area of concern. It is read as follows:
"Within 30 nautical miles of a line from North Bay to Muskoka to London."
The latitudes and longitudes should correspond to the locations issued. Weather systems rarely have the decency to occur right over well marked and known locations, so it is often the case where a reference might be issued such as "30 W of Fredericton", or "60 S of Gander". In such cases, the latitude and longitude pair will relate to the actual position, rather than being meant as "30 W of this point".

Line 4 is the description of the phenomenon. This part reads like this:
"Thunderstorms, with maximum tops to 30,000 feet, observed on radar. The line is moving eastward at 20 knots. Little change in intensity is expected throughout this SIGMET's period of validity."
For the case of thunderstorms, the higher the tops, the more violent the activity within. This relates to everything bad about thunderstorms, including turbulence, lightning, hail, everything. In the maritimes, we often see SIGMETs in the summertime warning of tops as high as FL450. I also know that our storms are weak by comparison to many areas. Other terms typically seen in SIGMETs mention conditions reported by pilots, observed on satellite "pix" for pictures, weather radar, and from the lightning detector network now deployed across North America. Other weather events have their own characteristics. Many are defined by a series of points, and then concluded with "thence to point of origin" or some words to that effect, bounding an area with a polygon. Terms like "WKNG" for "weakening" are used to describe the forecast trend in the intensity of the condition.

The description of phenomenon (I hate typing that word) is often the most confusing, since it will include more and more terms that are abbreviated. Like I said, with a little experience and the context already understood, these abbreviations are not too far from comprehensibility.




That's all there really is to SIGMETs. The information contained within them can be vital, as it often pertains to serious conditions. Pilots would do well to consider avoiding the areas described in most cases. Any comments, concerns or thoughts? Please e-mail me at moxner@nbnet.nb.ca. Thanks for taking the time to read.