How to Read a Graphical Area Forecast (GAF)
The forecast says scattered cloud at 4,000 feet. A few characters later, the base has dropped to 3,500 feet and the cloud is broken. Somewhere in the same area there is fog with just 300 metres of visibility. Which bit applies to your flight?
A Graphical Area Forecast becomes much easier to read when you follow the relationship between its map and its text. The abbreviations provide the detail, but your position and timing decide which detail you need. We will work through a real NSW-E example, including the small qualifications that can change your answer in a theory question.
The extracts below come from a historical forecast for 5 August 2026.
Contents
Start with the header and the clock
A GAF describes the weather over a broad area from the surface to 10,000 feet above mean sea level. Each forecast covers six hours; two consecutive forecasts are issued together to provide twelve hours of coverage. A longer trip may therefore need the next chart as well as the one covering departure.
Our example is labelled NSW-E and CORRECTED. Its issue group, 2608051054Z, means 5 August 2026 at 1054 UTC. The validity group, 051100 to 051700Z, means 1100 to 1700 UTC on the fifth day of the month. The issue time tells you when this version was produced; the validity tells you when its forecast conditions apply.
That distinction earns its place at the top of your working. At 1045 UTC, this forecast would still be describing a later period. At 1730 UTC, you would need the following period. Choosing a promising-looking row from the wrong forecast can leave every subsequent calculation perfectly neat and completely irrelevant.
For most of NSW in August, add ten hours for Australian Eastern Standard Time. This example runs from 2100 on 5 August to 0300 on 6 August. It is an overnight forecast. Keep the day change in your notes, and establish the applicable local offset before converting another example.
The separate weather-features time identifies the time represented by any depicted weather systems. Here it is 051100Z. Where a front or trough has a movement arrow, its printed position is a starting position. Read any movement information with the timing of your flight in mind.
Find your weather area before decoding it
The map contains large lettered areas and smaller subdivisions. Start by locating the part of the route named in the question, then identify every weather area it crosses. A route near a boundary deserves a look at the conditions on both sides.
In this example the main areas are A to E. B1 and B2 belong to Area B. The green scalloped boundaries separate forecast weather regions; the solid black outline marks the forecast's outer limit. These lines describe a forecast distribution. Weather encountered near them can vary, so avoid planning around a supposedly exact edge between good and poor conditions.
Read Area B first, then apply whichever qualifications refer to B1 or B2. A sub-area changes particular elements of its parent forecast. You still need the other applicable parent-area entries. Searching for a completely separate B1 forecast in the first column will send you round the page unnecessarily.
For an exam question, write the area or sub-area next to the relevant time. For flight planning, mark the transitions along your route. This gives each stretch of the flight its own weather context before you consider an altitude or a diversion option.
Read across Area B, then work down
The text is arranged so that visibility and weather sit beside the associated cloud information. Follow each horizontal section across before moving down to the next one. The dotted separators help you keep those relationships together.
The first Area B line has visibility greater than 10 kilometres and NIL significant weather. Beside it, SCT SC 4000/8000FT describes scattered stratocumulus with a base at 4,000 feet and tops at 8,000 feet AMSL. The bracketed B1 qualification changes the cloud there to broken, with bases at 3,500 feet. The stated top remains 8,000 feet.
SCT cloud covers three to four oktas, or eighths of the sky; BKN covers five to seven. The difference is useful when you picture how much sky may be occupied. Also notice the word BASES: the 3,500-foot figure changes the bottom of the layer. The layer therefore extends from its local base to the stated top.
Now read the poorer-visibility entries. Over land, isolated smoke is forecast below 8,000 feet, with visibility of 5,000 metres. A separate entry forecasts isolated fog over land with visibility of 0300M: 300 metres. That leading zero is worth a deliberate second look.
The empty cloud cells beside those entries leave the cloud already forecast in place. They avoid repeating unchanged cloud information. Nor does the first line promise ten-kilometre visibility everywhere in B. The remaining lines tell you where and why visibility can be much worse.
The final entry specifies moderate turbulence below 6,000 feet over land in B2. Keep that B2 restriction attached to the turbulence. The smoke and fog entries above apply over land in their parent area and carry no equivalent B2-only restriction.
Try reading the whole section aloud as a short briefing. If your explanation sounds like disconnected code words, return to the map and say where each condition belongs. That small change in study technique helps turn recognition into interpretation.
Follow Area C's weather and timing together
Area C offers a useful test of how carefully you connect a weather entry to its cloud and time qualification. First establish its prevailing cloud, then examine the conditions accompanying the poorer visibilities.
The prevailing cloud includes scattered stratocumulus from 1,000 to 3,000 feet, with broken cumulus or stratocumulus from 1,500 to 8,000 feet AMSL. Those low bases should already have your attention before you begin looking for precipitation.
The next section forecasts 3,000 metres visibility in isolated rain showers, with snow showers above 5,000 feet. Beside it are isolated towering cumulus based at 4,000 feet and extending above 10,000 feet, plus broken stratus from 2,000 to 4,000 feet. Keep the unchanged prevailing layers in the picture as well.
Below that, 2000M SCT DZ, ISOL FM 15Z means 2,000 metres visibility in scattered drizzle, becoming isolated from 1500 UTC. Its associated cloud includes broken stratus with a base at 0800FT, or 800 feet AMSL, and overcast stratocumulus from 3,000 to 8,000 feet.
Here SCT describes the geographical coverage of drizzle. Earlier, SCT SC described cloud amount in the sky. The same abbreviation serves different jobs, so read the weather or cloud type immediately beside it.
From 1500 UTC, the drizzle becomes less widespread. The entry gives no higher visibility value within the remaining drizzle and no lifting of its associated stratus base. It also leaves the separate isolated-fog forecast in place. A change to one element gives you permission to change that element in your answer.
If that distinction takes a few attempts, practise with one row at a time. Explain the conditions before the stated change, then explain them afterwards. PPL Prep's online ground school develops the meteorology behind these codes, so you can connect a forecast's wording to the weather it describes.
Put cloud and terrain on the same height scale
Every height on this GAF is AMSL. A base at 3,500 feet gives you its altitude above mean sea level. To understand its relationship to terrain, compare it with terrain elevations on that same datum.
TAFs and METARs use a different reference for their cloud groups: hundreds of feet above the aerodrome elevation. For a deliberately invented comparison, take an aerodrome at 1,500 feet with BKN020 in its TAF. The forecast base is 2,000 feet above that aerodrome, or 3,500 feet AMSL. That can be consistent with the B1 base in our GAF.
The arithmetic is straightforward once the reference is clear. Write AMSL or above aerodrome beside each figure before comparing them. Otherwise, a perfectly ordinary 1,500-foot difference can look like a disagreement between forecasts.
Use the result to assess the route against terrain and the applicable VMC requirements. A subtraction at one point cannot establish safe clearance along an entire valley or across a ridge. Build in room for forecast uncertainty and changes along the route, and consider your available escape options before departure.
Include freezing level, turbulence and the legend
Area B's freezing level runs from 5,000 feet in the southwest to 9,000 feet in the northeast. These are geographical variations within B. Select the relevant part of the area when considering your route. The southwest and northeast labels describe position.
Area C gives a freezing level of 5,000 feet. Its forecast cloud extends through that altitude, so the legend's icing implications belong in your interpretation. The legend states that cloud above the freezing level implies moderate icing; cumulus, stratocumulus and altocumulus imply moderate turbulence. Thunderstorms, cumulonimbus and towering cumulus imply severe icing and severe turbulence.
These implications are part of the forecast even when the individual row leaves them unstated. Reading the visible cloud codes while skipping the legend leaves out some of the most consequential information on the page.
For a VFR flight, use this information to recognise hazardous weather and preserve a route that remains within your qualifications and the aircraft's capabilities. A freezing-level figure provides weather information; it offers no assurance that flying in cloud below it is acceptable.
Read the remarks before putting the chart away
The remarks explain why this forecast was corrected: a duplicate reference to LAND was removed from Area B's smoke entry. The correction changes the wording of that entry without announcing a general improvement in the weather.
The same section contains forecasts for named locations where terrain can make the weather particularly restrictive. Look at Mount Victoria, shown as MVI. The printed elevation is 3,700 feet, and its forecast includes scattered stratocumulus at 4,000 feet AMSL. That places the stated cloud base only 300 feet above the listed site elevation.
Bowral, BWL, has an elevation of 2,200 feet and scattered stratocumulus at 3,000 feet AMSL, a difference of 800 feet. The arithmetic shows why a cloud altitude that seems generous at the coast can be restrictive inland. Surrounding terrain still needs its own assessment.
These entries resemble aerodrome forecasts, but the remarks explicitly retain AMSL heights. Use the printed cloud altitude directly when comparing it with the location's elevation. Read the label above the figures, especially when the layout tempts you to use a familiar TAF habit.
Cross-check the GAF with the rest of the briefing
Use the GAF to develop the en-route picture, then examine the departure and destination TAFs for conditions expected around those aerodromes at the relevant times. Compare the latest METARs and SPECIs with what was forecast, allowing for the age and limitations of the observations. A clear observation at one airport cannot describe the whole route or promise what will happen at arrival.
Check applicable AIRMETs and SIGMETs as well. AIRMETs report specified deterioration missing from the GAF, including relevant changes in location or timing. SIGMETs cover specified hazardous en-route phenomena. Adverse conditions already forecast in the GAF still need to be accounted for when no warning is issued.
Obtain the appropriate grid point wind and temperature forecast for your planned level and time. The GAF's weather-feature movement information is a different quantity from the wind you need for heading, groundspeed and fuel calculations.
When information appears inconsistent, first check the dates, areas and height references. Then investigate the remaining difference using the current briefing and appropriate weather advice. Resolving that uncertainty on the ground gives you far better options than discovering it while approaching a lowering cloud base.
Test yourself on the example
Before looking back through the guide, use the extracts to answer these questions. They are original practice prompts based on this forecast.
In B1, what are the forecast stratocumulus amount and base?
Does the Area B fog entry mean 300 or 3,000 metres visibility?
What changes in Area C from 1500 UTC?
How far above its listed elevation is the forecast cloud base at Mount Victoria?
The answers are broken stratocumulus with a 3,500-foot AMSL base; 300 metres visibility; drizzle coverage becoming isolated; and a 300-foot difference at Mount Victoria. For the third answer, keep the stated drizzle visibility and associated cloud unchanged. For the fourth, remember that the calculation describes one location.
On your next practice chart, explain one stretch of route in ordinary language before checking the answer. You should be able to point to the row behind each statement and identify any qualification that changes it. The page becomes much less intimidating when every number has a place and a time.
PPL Prep's online ground school covers the meteorology that helps you make sense of these GAF rows, including the cloud and visibility behind the abbreviations. The video below shows how the course works and introduces the teaching approach you can use for your next weather-study session.
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