MANAIR Manual of Standards and Procedures for Aviation Forecasts: chapter 4


4.1 Definition

A graphic area forecast (GFA) consists of six charts:

  • two valid at T0 +0 hr (the beginning of the forecast period);
  • two valid at T0+6 hr (six hours into the forecast period); and
  • two valid at T0+12 hr (the end of the forecast period).

Of the two charts valid at each time, one shows the clouds and weather portion; the other shows the icing, turbulence and freezing level for the same time and area.

In addition, the clouds and weather chart of the last set also contains a worded instrument flight rules (IFR) outlook extending over the next 12-hour period. For a full description of these charts, refer to 4.9 and C.2 of Appendix C.

4.2 Purpose

Ascent and descent planning information for higher performance aircraft is an important but secondary function of the GFA. The GFA is designed primarily to meet general aviation and regional air carrier requirements for pre-flight route planning in Canada. Providing each panel graphically describes the most probable meteorological conditions expected to occur between the surface and 24,000 feet over a given area at a specified time.

4.3 Issue and valid times

A GFA is issued to reach users approximately half an hour before the beginning of the forecast period (that is, half an hour before T0).

A GFA is issued at 2330, 0530, 1130 and 1730 UTC, and are valid at 0000, 0600, 1200 and 1800 UTC respectively. Each GFA covers a period of 12 hours with an IFR outlook for the next 12.

4.4 Domain

There are seven GFA domains which cover the entire domestic airspace as well as a portion of Gander oceanic airspace for which Canada has the responsibility for the provision of air traffic control services. Refer to C.1 of Appendix C for the map of the seven GFA domains.

4.5 Units

Units to be used in the GFA are the conventional ones used in North America as listed:

  • horizontal distance: measured in nautical miles (NM)
  • speed: measured in knots (KT)
  • atmospheric pressure: measured in hectopascals (hPa)
  • wind direction: measured in degrees true
  • horizontal visibility: measured in statute miles and fractions of statute miles (SM)
  • cloud height: measured in hundreds of feet above sea level (ASL), with the exception of ceiling which is above ground level (AGL)
  • base and top of areas of icing and turbulence: measured in hundreds of feet ASL
  • freezing level: measured in hundreds of feet ASL
  • time: measured UTC (Z) with midnight indicated by the numerals zero and zero "00"

4.6 Map projection, scale and background

As per the Technical Regulations, Volume II, Meteorological Service for International Air Navigation, section [C.3.3] 3.2.2 (WMO-No. 49), the map shall use the polar stereographic projection true at 60 degrees latitude north.

For clarity, the map background contains the following features:

  • provincial, federal and territorial boundaries
  • oceans, coastlines and major lakes
  • Canadian airports for which a TAF is issued (indicated as a black circle)

The scale of the map is chosen to allow the map to fit in the appropriate space on the chart. All charts include a scale bar in their corresponding legend box to help users determine approximate distances on the map.

4.7 Characters, abbreviations and symbols

The number and type of characters, abbreviations and symbols allowed in the GFA are limited to those familiar to and fully understood by all users of the product. These are described in the TP 14371, Transport Canada Aeronautical Information Manual (TC AIM), and they are already used in other graphical products (such as SIG WX prognoses) currently issued for the aviation industry.

4.7.1 Abbreviations

A GFA is prepared in abbreviated English language using approved MSC abbreviations as published in the Manual of Word Abbreviations (MANAB). If an abbreviation is not defined, plain English shall be used.

4.7.2 Symbols

The following is the list of symbols allowed in a GFA:

  • fronts (cold, warm, stationary, surface and upper)
  • trowals
  • high and low pressure centres
  • frontal waves
  • surface and upper troughs
  • direction-of-motion arrows
  • freezing rain and freezing drizzle
  • ice pellets
  • wind barbs
  • icing (moderate and severe)
  • turbulence (moderate and severe)
  • thunderstorms
  • tropical cyclones (storm and hurricane)
  • volcanoes
  • radioactive releases

Refer to C.2 of Appendix C for examples of symbols.

4.8 Page layout

The page layout displays the title box, the legend box and the comments box in a column along the right-hand side of the chart. The remainder of the page depicts significant clouds and weather or icing, turbulence and freezing level.

4.8.1 Title box

The title box must include: chart name; four-letter ID of the centre transmitting the charts (CWAO refers to the Canadian Meteorological Centre/Network Operations (CMC/NETOPS)); date and time of issue; the name of the GFA region; chart type; and valid date and time of the chart. This box must be placed in the upper right-hand corner of the chart.

Example of a title box for a clouds and weather chart:

Title box 1. See long description below.

Description of image

Box shows sample data: GFACN31 CWAO, followed by Pacific Region/Region du Pacifique, followed by Clouds and Weather/Nuages et Temps. Lower half of the box shows date of issue in format 09/03/2012 2331Z, with last line showing VLD 10/03/2012 1200Z

Example of a title box for an icing and turbulence chart:

Title box 2. See long description below.

Description of image

Box shows sample data: GFACN31 CWAO, followed by Pacific Region/Region du Pacifique, followed by ICG TURB & FZLVL. Lower half of the box shows date of issue in format 09/03/2012 2332Z, with last line showing VLD 10/03/2012 1200Z

4.8.2 Legend box

The legend box includes the less common symbols used in the depiction itself. In addition, this box must include a scale bar to help users determine distances on the chart. This box is as wide as the title box and is located immediately below it. Just underneath the legend box are the following three standard phrases:

HGTS ASL UNLESS NOTED
CB TCU AND ACC IMPLY SIG TURB AND ICG
CB IMPLIES LLWS

Example of a legend box for a clouds and weather chart:

Legend box 1. See long description below.

Description of image

Legend box for a clouds and weather chart

Example of a legend box for an icing and turbulence chart:

Legend box 2. See long description below.

Description of image

Box shows, in order, title LEGEND, FZLVL, symbols for icing and turbulence, a scale bar, then the three phrases. Default information on heights and weather phenomena is at bottom.

4.8.3 Comments box

The comments box provides any information that forecasters consider important (such as formation or dissipation of fog that decreases or increases the visibility from one category to another).

The box may also be used to describe elements that, if added to the depiction, would clutter the chart. Furthermore, the comments box of the T0+12 hr clouds and weather chart also includes a 12-hour IFR outlook statement.

Example of a comment box for a clouds and weather chart:

Comment box 1. See long description below.

Description of image

Box provides information considered important to the clouds and weather chart, in code format,. Eg., A: 70-80 OVC 180 INTM 3-6SM -SN/-RA BR PTCHY CIGS 5-15 AGL; B: 60 SCT 120 P6SM ISOLD TCU 150 GVG 5SM -SHRA BR; C: 20-40 BKN 120 P6SM ISOLD TCU 140 3SM -SHRA CIGS 10 AGL

Example of a comment box for an icing and turbulence chart:

Comment box 2. See long description below.

Description of image

Box provides information considered important, in code format, to the icing and turbulence chart.

4.9 Content of the GFA

A description of the content of the GFA is given in this section. Examples of GFA content are given in C.2 and C.4 of Appendix C.

4.9.1 Clouds and weather chart

The clouds and weather chart includes clear and concise information on cloud layers and surface-based phenomena, visibility, weather and obstructions to vision at the valid time of the chart. In addition, this chart includes all relevant synoptic features present and responsible for the weather, indicating their speed and direction of motion at the valid time.

This chart must contain the following:

  1. Title box
  2. Legend box
  3. Comments box, which includes the IFR outlook only on the T0+12 hr depiction
  4. CLDS and WX indicating:
    • synoptic features
    • speed and direction of motion of synoptic features at the valid time
    • organized areas of clouds
    • organized areas of precipitation
    • organized areas of obstruction to vision
    • visibility, weather and obstructions to vision
    • isobars
    • strong surface winds and gusts
    • IFR outlook only on the T0+12 hr chart

GFA Clouds and Weather Chart. See long description below.

Description of image

Layout of the information on chart. Left, large pane displays cloud and weather information. Smaller panes on right show title box, legend box, comments box.

Figure 1: GFA clouds and weather chart

4.9.1.1 Synoptic features

Synoptic features, such as lows, highs, fronts and troughs are indicated with the corresponding symbolic representation used in surface analyses. When the motion of a main synoptic feature is expected to be five knots or more, it is indicated by an arrow for the direction, with the speed in knots enclosed in a box as follows.

Example: Synoptic features

Example: Synoptic Feature. See long description below.

Description of image

Depiction of low pressure centre along showing motion (direction and speed). Black arrow points to the right, indicating direction. Tip of arrow shows box with the figure 10 inside, indicating speed in knots.

When the motion of a main synoptic feature is expected to be slower than five knots, the term QS, for quasi-stationary, is used.

4.9.1.2 Clouds
4.9.1.2.1 Cloud area and amount

A brown scalloped border encloses organized areas of broken (BKN) or overcast (OVC) cloud between the surface and 24,000 feet. However, when convective clouds are expected (such as TCU, ACC and CB), tops are stated, when known, even if they extend above the 24,000-foot level. Significant cloud layers are based on the amount at each level, not on the summation amount. They are stated using the abbreviations allowed by the Manual of Word Abbreviations (MANAB). Altitudes of bases and tops of all layers are given, together with the amount. Cirrus clouds are not mentioned in the GFA. Sky conditions are indicated alphanumerically (e.g. SCT).

Abbreviation Definition
SKC Sky completely free of cloud
FEW Cumulative coverage is 1-2 oktas inclusive
SCT Cumulative coverage is 3-4 oktas inclusive
BKN Cumulative coverage is 5-7 oktas inclusive
OVC Cumulative coverage is 8 oktas

In areas where there are no organized clouds and the visibility is expected to be greater than six statute miles (P6SM), the terms SKC, FEW or SCT (whichever one is appropriate) are also used.

Example of organized broken clouds with embedded convective clouds:

Example. See long description below.

Description of image

Rough circle with brown scalloped border, with coded description of cloud inside: BKN 40/100 P6SM ISOLD TCU 120 P6SM -SHRA.

4.9.1.2.2 Cloud type

Cloud type is not indicated with the exception of convective clouds (CU, TCU, ACC and CB) which are always indicated regardless of amount.

4.9.1.2.3 Cloud base and top

The height of both the base and top of clouds are indicated. All heights are stated in hundreds of feet (for example, 2 means 200 feet, 40 means 4000 feet). Unless otherwise specified, all heights in the depiction are the vertical distances above sea level (altitudes). This rule is defined on all graphic charts by including the note HGTS ASL UNLESS NOTED at the bottom of the legend box.

When, in a particular case, heights are given with respect to ground level instead of sea level, this situation is clearly stated by using the ceiling designator CIGS with the indicator AGL (above ground level).

Example: LCL CIGS 5 AGL

In mountainous areas, the ceiling designator should be used with caution. Instead, when low layers of cloud are expected to obscure portions of mountainous terrain, statements such as HIER TRRN OBSCD, MTS OBSCD ABV 35, MT TOPS OBSCD, and ALL PASSES CLSD may be added to emphasize the expected conditions.

Ranges of variability in the altitudes of cloud bases should not be stated when the bases are expected to be 2,500 feet or more above the highest terrain in the area. Bases and tops are stated for all cloud layers, provided there is a minimum spacing of 2,000 feet between cloud layers.

When convective clouds are expected (such as TCU, ACC and CB), tops are stated, even if they extend above the 24,000-foot level. These tops being usually quite variable only the upper limit of the convective clouds is forecast, as well as the most likely time for significant development to start, for instance, CB 300.

By convention, the base and top of clouds are indicated immediately in the form of a fraction whose denominator is the base and the numerator is the top of the cloud, both in hundreds of feet above sea level. When an organized area of clouds exists in layers with little separation, then the term LYRS can be used to describe them.

Example of cloud description:

Example of cloud description

4.9.1.2.4 Convective clouds

As mentioned in 4.9.1.2.3 unlike other types of cloud, convective cloud tops are always explicitly stated, even when they extend above the 24,000-foot level. The type of convective cloud is also stated, regardless of whether or not it is the main cloud or embedded within another cloud deck. As well, if CBs are present with thunderstorms, the appropriate symbol (TS) is added to the area. In dealing with convective clouds and the precipitation associated with them, keep in mind that a qualifier may be used to specify the amount of cloud and another qualifier to refer to the visibility associated with the precipitation generated by the convective cloud. However, in the description ISOLD TCU 180 GVG 2SM -SHSN, the descriptor ISOLD is not repeated for the SHSN because it is understood and implied by the fact that the TCU are isolated. In those cases when ISOLD, OCNL or FRQconvective clouds are embedded in a larger area of clouds, the base of the clouds is not stated, only the top. However, if only convective cloud is present, both the base and the top must be stated, as would be with non-convective cloud.

The following qualifiers regarding convective clouds and associated showers are used in a GFA according to the following spatial coverage definitions when the cloud is in association with another main cloud description.

Abbreviation Description Coverage
ISOLD Isolated 25% or less
OCNL Occasional Greater than 25% and up to 50%
FRQ Frequent Greater than 50%

Under certain circumstances, a spatial qualifier is used without associating it to an enclosed area. In such a case, a reference is made to the geographical location where the phenomenon is expected to occur

Example: OVR XTRM NRN SXNS ISOLD TCU 180 P6SM -SHRA

4.9.1.2.5 Surface-based layers

Surface based layers are described by stating the vertical visibility in hundreds of feet.

Example (1): OVR/VC LK SUPR LCL CIGS 0 AGL

If the top of the surface-based layer is well defined, then it is stated in a similar fashion.

Example (2): XTNSV CIGS 0-2 AGL TOPS 15

4.9.1.3 Weather, visibility and obstruction to vision

The following qualifiers regarding restriction to visibility, non-convective precipitation and ceilings may be used in a GFA according to the following spatial coverage definitions.

Abbreviation Description Coverage
LCL Local 25% or less
PTCHY Patchy Greater than 25% and up to 50%
XTNSV Extensive Greater than 50%

The percentage of the area coverage specified by a qualifier refers to the area where the actual weather phenomenon is expected to occur. For instance, the following example means that more than 50% of the area enclosed within the dashed orange lines is forecast to have a visibility lowered to 1/4-1SM by mist or fog.

Example of lowered cloud and visibility due to mist or fog:

Example 2. See long description below.

Description of image

Depiction of an area of restricted visibility and its associated ceilings. Oval shaped area is enclosed by a dashed dark orange line with coded description WTN DASHED LN XTNSV 1/4 - 1SM FG/BR CIGS 1-4 AGL.

Alternatively, a spatial qualifier may be used without associating it to an enclosed area. In such case, a reference is made to the geographical location (e.g. over SWRN SK) where the phenomenon is expected to occur.

4.9.1.3.1 Precipitation

1) Continuous precipitation: Areas of continuous precipitation are enclosed in continuous green lines with the inside area stippled. Red is used instead of green for freezing precipitation. The description of the precipitation and the visibility is stated inside the area.

Example (1): Continuous precipitation

Example 1- continuous precipitation. See long description below.

Description of image

Oval shaped area enclosed by a solid green line with the inside stippled with green dots and the coded description 1-3SM -SNRA BR.

Example (2): Continuous freezing precipitation

Example 1- continuous freezing precipitation. See long description below.

Description of image

Oval shaped depiction of an area of continuous freezing, enclosed by a solid red line with the inside stippled with red dots. Inside is the coded description 3-6SM -FZRA BR.

2) Contiguous areas of different types of continuous precipitation: An area of continuous precipitation containing marked changes in precipitation types is separated by either a solid green or red line depending on the precipitation type change.

Example: Contiguous areas of different types of continuous precipitation

Example 3. See long description below.

Description of image

Circular shape featuring three layered areas - middle area is an area of freezing rain depicted by an enclosed solid red line with the inside stippled with red dots. Inside is the coded description 4-P6SM -FZRA BR. This area is depicted along and to the north of a warm front. A second area, contiguous and north of the area of freezing rain, is depicted by a solid green line with the inside stippled with green dots. Inside is the coded description (1-3SM -SN). A third area, contiguous and south of the area of freezing precipitation (south of the warm front as well) is depicted by a solid green line with the inside stippled with green dots. Inside is the coded description (3-6SM -RA BR).

3) Convective or intermittent precipitation: Areas of convective or organized but intermittent precipitation are enclosed in a dashed green line with the area hatched. The description of the precipitation and the visibility is stated inside the area.

Example: Convective or intermittent precipitation

Example 4. See long description below.

Description of image

Oval shaped area enclosed by a dashed green line with the inside hatched by green diagonal lines with a positive slope. Inside is the coded description FRQ TCU180 4-P6SM -SHRA BR.

An area of intermittent precipitation containing marked changes in precipitation types is treated in a manner similar to continuous precipitation. The only difference is the use of a dashed line to separate the precipitation types.

4) Disorganized precipitation: Disorganized precipitation is indicated alphanumerically without the use of an enclosed area, either at the location of expected occurrence or by specifying the location. For example:

ALG  TROF  ISOLD  TCU  150 P6SM  -SHRA
ONSHR  FLO  LK  HURON  OCNL  TCU  150  4-P6SM  -SHSN
IN  VLY  LCL  4-SM  -FZRA  BR

4.9.1.3.2 Visibility

The visibility, in statute miles, is always included. When the visibility is expected to be greater than six miles, it is indicated as P6SM.

4.9.1.3.3 Obstruction to vision

A significant area with visibility restricted to six statute miles or less is depicted by an enclosed dashed orange line with the description stated inside the area or in the comment box. If this area is also affected by precipitation, it is not depicted separately because the obstruction to vision is already included in the precipitation group (e.g. 1/2-2SM  -RA FG/BR).

Example: Obstruction to vision dipiction

Example: obstruction to vision depiction. See long description below.

Description of image

Oval shaped area showing depicting visibility to 6 statute miles or less. Area enclosed by a dashed orange line. Inside is the coded description 1/4-1SM FG/BR.

For unorganized obstruction to vision, a description is either put directly on the chart or in the comments box.

Example: ALG  CST  LWR  N  SHR  QC  LCL  3/4SM  BR

Smoke from forest fires

The main aviation hazard posed by forest fires is reduced visibility due to smoke. The actual and forecast extent of the area affected by smoke from a forest fire can be difficult to determine. A statement about reduced visibility due to smoke is included in the following comment.

Example: PTCHY  3-5SM  FU  DNWND FOREST FIRES E OF PRINCE RUPERT EXC LCL  1/2SM  FU  VC  FOREST FIRES.

The extent of the area affected by reduced visibility due to smoke from a forest fire may also be depicted by an orange dashed line used for obstructions to vision.

4.9.1.4 Isobars

Isobars are included on the clouds and weather chart only. They are depicted as solid black lines with units in hectopascal (hPa), drawn every four hectopascal starting at 1000 hPa  as a reference.

4.9.1.5 Strong surface winds

The direction and speed of strong surface winds are indicated by using wind barbs for wind speed and direction for all areas where winds are expected to have a mean sustained speed of at least 20 KT. Wind gusts are indicated when speeds of at least 10 KT greater than the mean sustained winds are forecast. The gusts are indicated by the letter "G", followed by the gust speed in knots, enclosed in a box, as indicated by the following symbol.

Strong Surface Winds. See long description below.

Description of image

Wind barb illustrating strong surface winds, where stem indicates the direction, while three feathers indicate speed. Box above the stem contains text to indicate wind gusts: G35.

When accompanied by strong gusts, mean sustained winds of less than 20 KT may also be indicated in the GFA, at the forecaster’s discretion, if moderate mechanical turbulence is expected to occur as a result of the wind gusts.

4.9.1.6 IFR outlook

The outlook is included in the comments box as part of the T0+12 hr clouds and weather chart. It should be fairly general and indicate the main areas where IFR weather is expected. In addition, it should state the cause of the IFR weather, along with weather hazards, if any. For example, if IFR ceilings were expected over eastern New Brunswick, the IFR outlook would say CIG/VIS  RA  BR  OVR  ERN  NB.

4.9.1.6.1 Tropical cyclones

Tropical cyclones, when expected in a GFA domain, will be described following the guidance of the Canadian Hurricane Centre of the MSC in collaboration with the designated ICAO Tropical Cyclone Advisory Centre. Examples of tropical cyclone depiction are found in C.2 of Appendix C.

4.9.1.6.2 Volcanic ash

Volcanic ash, when expected in a GFA domain, will be described following the guidance of a designated ICAO Volcanic Ash Advisory Centre (VAAC).

Volcanic ash is not depicted in a GFA. When it is expected within a GFA domain, it is mentioned in the comment box of the CLDS & WX CHART in the following format.

VOLCANIC ERUPTION

NAME OF VOLCANO - COUNTRY

LOCATION OF VOLCANO - (LAT/LONG IN DEGREES AND TENTHS OF A DEGREE)

FIRST ERUPTION (xxxxZ day/month/year)

STATUS OF THE ERUPTION (CONTINUING OR ENDED)

CHECK SIGMETS FOR VOLCANIC ASH POSITION

Example: Volcanic ash comment box

VOLCANIC ERUPTION

EYJAFJALLAJOKULL - ICELAND

N6337 W01937

FIRST ERUPTION 1826Z 14/04/2012

CONTINUING

CHECK SIGMETS FOR VOLCANIC ASH POSITION

Additionally, if the volcano at source of the event lies within a GFA domain, a volcanic eruption symbol is placed at the approximate location of the volcano on the chart. An example of the volcanic eruption symbol is found in C.2 of Appendix C.

4.9.1.6.3 Radioactive cloud (release)

Radioactive cloud (release), if expected in a GFA domain, will be described following the guidance of the Canadian Meteorological Centre of the MSC which is a designated WMO Regional Meteorological Specialized Centre (RMSC).

Radioactive cloud (release) is not depicted in a GFA, but, when expected within a GFA domain, is mentioned in the comment box of the CLDS & WX CHART in the following format:

RADIOACTIVE RELEASE

NAME OF SITE - COUNTRY

LOCATION OF SITE - (LAT/LONG IN DEGREES AND TENTHS OF A DEGREE)

FIRST RELEASE (xxxxZ day/month/year)

STATUS OF RELEASE (CONTINUING OR ENDED)

CHECK SIGMETS FOR RADIOACTIVE CLOUD POSITION

Example: Radioactive cloud release comment box

RADIOACTIVE RELEASE

DARLINGTON, ON. CANADA

N4352 W07843

FIRST RELEASE 1826Z 14/04/2012

ENDED

CHECK SIGMETS FOR RADIOACTIVE CLOUD POSITION

Additionally, if the radioactive release source lies within a GFA domain, a radioactive release symbol is placed at the approximate location of the source on the chart. An example of the radioactive release symbol is found in C.2 of Appendix C.

4.9.2 Icing, turbulence and freezing level chart

The icing, turbulence and freezing level chart is used to depict areas of moderate to severe non-convective icing and non-convective turbulence, along with the type, intensity, and base and top of layers. Otherwise, a general statement about icing and turbulence found in the comments box is applicable.

This chart must contain the following:

  1. Title box
  2. Legend box
  3. Comments box
  4. Icing (ICG), turbulence (TURB) and freezing level (FZLVL) indicating:
    • synoptic features described in the clouds and weather chart
    • speed and direction of motion of synoptic features at the valid time
    • areas of icing
    • areas of turbulence and low level wind shear
    • freezing level contours

Chart. See long description below.

Description of image

Chart showing a large pane on the left, to display the icing, turbulence and freezing level information, using the coded description ICG-TURB-FZLVL. Three smaller panes are stacked on the right side. Top pane displays title box, centre pane displays legend box, bottom displays comments box.

Figure 2: GFA icing, turbulence and freezing level chart

The following qualifiers regarding icing and turbulence may be used in a GFA according to the following special coverage definitions:

Abbreviation Description Coverage
LCL Local 25% or less
PTCHY Patchy Greater than 25% and up to 50%
XTNSV Extensive Greater than 50%
4.9.2.1 Icing (ICG)

Areas of moderate or severe non-convective icing are depicted by an enclosed continuous blue line with coarse stippling in blue. The intensity and type of icing, as well as the base and top of the icing layer is described in text and by means of their corresponding symbols.

Symbol indicates moderate icing:

Symbol: indicates icing. See long description below.

Description of image

Symbol indicating moderate icing depicted by the lower half of a semicircle, with two vertical lines crossing the bottom half of the semi circle.

Symbol indicates severe icing:

Symbol: indicates severe icing. See long description below.

Description of image

Symbol indicating severe icing depicted by the lower half of a semicircle, with three vertical lines crossing the bottom half of the semi circle.

The base and the top of the icing layer is in the form of a fraction, the denominator of which represents the base and the numerator representing the top of the icing layer in hundreds of feet ASL.

Symbol indicates that the base of the icing layer is at the surface while the top is at 2,000 feet above sea level:

Fraction. See long description below.

Description of image

Fraction format of icing layer, showing 20 as the numerator and SFC as the denominator.

The types of icing are: RIME, MXD (mixed) or CLR (clear).

Example of a depiction of the types of icing:

Oval shaped depiction of an area of icing 1. See long description below.

Description of image

Oval shaped depiction of an area of icing, enclosed by a solid blue line with the inside stippled with blue dots. Inside is the coded description: a moderate icing symbol with the type of icing indicated below (MXD). The base and top of the icing layer is indicated to the right of the symbol 30/80.

Areas of severe icing are indicated with a denser stippling. When an area of severe icing is contained within another area of moderate icing, the area of severe icing will be depicted with a denser stippling as indicated in this example:

Oval shaped depiction of an area of icing 2. See long description below.

Description of image

Oval shaped depiction of an area of icing. Outer border enclosed by a solid blue line with the inside stippled with blue dots. Inside is the coded description: a moderate icing symbol with the type of icing indicated below (RIME). Base and top of the icing layer FZLVL/160 is indicated to the right of the symbol for moderate icing. A second circular area is depicted within the first, enclosed by a solid blue line with the inside stippled more densely with blue dots. Inside is the coded description: a severe icing symbol with the type of icing indicated below (RIME). Base and top of the icing layer FZLVL/140 is indicated to the right of the symbol.

Light icing is indicated in the comment box rather than on the chart itself. If icing is forecast to occur during only part of the GFA period, the predicted time of occurrence of the icing is indicated in the comments box of the appropriate chart.

Icing intensity is indicated according to the rate of ice accretion. Since no satisfactory instrument has been installed on commercial aircraft to directly measure the rate of ice accretion on an airframe, the following expressions must be interpreted qualitatively, according to the effect of ice formation on the flying characteristics of the aircraft, as described next.

Light (LGT) icing: The rate of ice accretion is such that flying for prolonged periods (over one hour) without using de-icing equipment may create a problem. Occasional use of de-icing or anti-icing equipment removes or prevents ice accretion. If de-icing or anti-icing equipment is used, no problem occurs.

Moderate (MDT) icing: The rate of ice accretion is such that even short encounters become potentially hazardous. De-icing or anti-icing equipment must be used or a diversion is necessary.

Severe (SEV) icing: The rate of ice accretion is such that de-icing or anti-icing equipment fails to reduce or control the hazard. Immediate diversion is necessary.

4.9.2.2 Turbulence (TURB)

Areas of moderate or severe non-convective turbulence are depicted by an enclosed continuous red line with coarse cross-hatching. The intensity and type of turbulence, as well as the base and top of the turbulent layer is described in text and by means of their corresponding symbols.

Symbol indicates moderate turbulence:

Depiction of Turbulence 1. See long description below.

Description of image

Symbol for moderate turbulence, in the form of a chevron pointing up.

Symbol indicates severe turbulence:

Depiction of Turbulence 2. See long description below.

Description of image

Symbol for severe turbulence, in the form of a double chevron pointing up.

The base and the top of the turbulence layer is in the form of a fraction, the denominator of which represents the base and the numerator representing the top of the turbulence layer in hundreds of feet ASL.

The following symbol indicates that the base of the turbulence layer is 18 000 feet, while the top is at 26 000 feet, above sea level:

Depiction of Turbulence 3. See long description below.

Description of image

Fraction format of turbulence layer, showing 260 as the numerator and 180 as the denominator.

The types of turbulence are:

  • clear air turbulence (CAT)
  • mechanical (MECH)
  • lee waves (LEE WV)
  • low level jet (LLJ)
  • low level wind shear (LLWS)

Lower level turbulence is depicted by the use of positive cross-hatching while higher level turbulence, by the use of negative cross-hatching.

Example: High and low level turbulence

Example, high and low level turbulence. See long description below.

Description of image

Two distinct areas of turbulence depicted as two partially overlapping ovals. First area is enclosed by a solid red line with the inside hatched by red diagonal lines with a positive slope. Inside is the coded description: a moderate turbulence symbol with the type of turbulence indicated below (MECH). Base and top of the turbulence layer SFC/30 AGL is indicated to the right of the symbol for moderate turbulence. Second area is depicted in similar fashion except for the negative slope of red diagonal lines. Inside is the coded description: a moderate turbulence symbol with the type of turbulence indicated below (CAT). The base and top of the turbulence layer 220/320 is indicated to the right of the symbol.

Severe turbulence is depicted with dense cross-hatching. The following example shows the severe turbulence surrounded by a larger area of moderate turbulence:

Example: Severe turbulence

Example, severe turbulence. See long description below.

Description of image

Two areas of turbulence depicted as two oval shapes, one within the other. The larger, outer area is enclosed solid red line with the inside hatched by red diagonal lines with a positive slope. Inside is the coded description: a moderate turbulence symbol with the type of turbulence indicated below (MECH). The base and top of the turbulence layer SFC/40 AGL is indicated to the right of the symbol. The second, smaller oval-shaped area is within the first, enclosed by a solid red line with the inside hatched more densely by red diagonal lines with a positive slope. Inside is the coded description: a severe turbulence symbol with the type of turbulence indicated below (MECH). The base and top of the turbulence layer SFC/30AGL is indicated to the right of the symbol.

Light turbulence is indicated in the comment box rather than on the chart itself.

The intensity of the turbulence is defined according to its effects on flying, as follows:

Light (LGT) turbulence: Such turbulence momentarily causes slight erratic changes in altitude and/or attitude (pitch, roll, and yaw).

Moderate (MDT) turbulence: Such turbulence is similar to light turbulence but of greater intensity. Changes in altitude and/or attitude occur, but the aircraft remains in positive control at all times. This turbulence usually causes changes in indicated air speed.

Severe (SEV) turbulence: Such turbulence causes large abrupt changes in altitude and/or attitude. It usually causes large variations in indicated air speed and the aircraft may be momentarily out of control.

A low level jet (LLJ), when reaching 50 KT or more is included in the GFA. It may be included at speeds between 35 KT and 45 KT, when upon the forecaster’s judgment, significant turbulence or shear is expected. LLJ is rounded to the nearest 5 KT and is depicted as follows:

Lower level jet. See long description below.

Description of image

Double-line arrow with a solid black tip. In a box overlapping the arrow is the code for low level jet and its speed of 60KT.

4.9.2.3 Freezing level (FZLVL)

Freezing level contours are indicated on a GFA by dashed lines. The height of the freezing level is indicated using the standard heights in hundreds of feet above sea level (e.g. SFC, 25, 50, 75, 100, etc., meaning, surface, 2,500 feet, 5,000 feet, 7,500 feet, 10,000 feet, etc.).

When more than one freezing level is forecast in the vertical, only the lowest level needs to be indicated, unless meteorological conditions are expected to be relevant to aviation safety (for example, freezing precipitation aloft). An above freezing layer (AFL) is depicted in a defined area as shown:

Freezing level (FZLVL). See long description below.

Description of image

Three black dashed lines representing the height of the freezing level, with three small text boxes at the end of each line, each containing SFC, 25 and 50, from the top dashed line to the bottom. A fourth line bulges in a semicircular shape from the surface (SFC) contour. This area represents a layer where the temperature is above the freezing point. Inside the area is the coded description for above freezing level (AFL) with the base and top of the layer 30/50 presented in fraction format.

Temporal changes in the freezing level, when significant, should be indicated in the comment box of the chart: FZLVL  20  LWRG TO SFC AFT 03Z

4.10 Amended GFA

Once issued, a SIGMET or AIRMET message automatically amends the current and relevant GFA. The remark (RMK) found in the National version of those messages indicate the GFA region(s) the SIGMET or AIRMET applies to (refer to 5.8.3.8 and 6.8.3.8).

4.11 Corrected GFA

A correction to a GFA is issued for the following events:

  1. Any unforecast weather phenomena not requiring an AIRMET (i.e. below AIRMET criteria threshold) or any other unforecast weather phenomena that, according to the forecaster, should be depicted in the GFA.
  2. Forecast weather phenomena (in the GFA) that fails to occur:
    • removal of forecast weather phenomena in the GFA chart that are no longer occurring or no longer expected to occur.
  3. A significant error made in a GFA chart:
    • a significant error is one which, if uncorrected, would result in an erroneous interpretation of the GFA and create a potential hazard to aviation.

For detailed guidance on GFA correction, refer to C.3 of Appendix C. Information about the nature of the correction made to the chart is included in the comments box.

Corrections are issued with the term CCX at the end of the first line of the title box, as in the following example:

GFACN31 CWAO CCX

In the above, X is a letter that may range from A to Z. The first correction would be "CCA", the second "CCB", the third "CCC" and so on.

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