The RouteWX guide to what a TAF is and how to read one.

• What is a TAF?
• How to read a TAF
• Wind
• Visibility
• Weather / Precipitation
• Cloud
• Changes to the weather
• Regional Variations
• Glossary of TAF Terms

What is a TAF?

TAF, or Terminal aerodrome forecast, is a weather report used in aviation to describe the weather forecast for a given airport. These reports are issued at set times throughout the day, specifically 0000z, 0600z, 1200z and 1800z and have a validity period of either 24 hours or 30 hours, with most civilian airports being 24 hours.

The report covers the weather that is forecast at the airport and in the surrounding area, usually up to 5 miles from the airport centre. Depending on which country the airport is located in, the TAF reports will be produced by different people. In some countries the report will be produced locally at the airport, in other countries a central weather service will be responsible for generating the report taking into account observations made at the airport.

Due to the increased complexity and understanding in weather forecasting rather than weather observing and reporting as is seen in a METAR report. Less airports will have TAF reports generated, because of this it is not uncommon for general aviation pilots to use a TAF report for a nearby larger airport that does issue TAF reports, rather than the actual smaller airport or airfield they intend to fly too. If the distance between the two airports is small it is very likely the TAF report will be fairly accurate for the smaller nearby airport too.

Military and mixed military/civilian airports also issue TAF reports, however the military issued TAF may have a shorter or longer validity period of up to 30 hours and may be issued more often than a civilian only airport report, especially if specific operations are going to take place at set times that require more accurate reporting.

Any report can be amended after it is issued if significant changes are now forecast beyond what the original report detailed.

Regardless of who produces the TAF they will all be written to follow a defined standard, allowing a pilot anywhere in the world to quickly read the report and build an understanding of the weather that may affect a flight to a specific airport.

How to read a TAF?

Each TAF report will cover a time range and detail the underlying weather to be expected for the entire time of the report. Any changes within this time period will be detailed, either as a temporary change for a set block of time, or as a change to the fundamental weather to be expected from the time of the change until the end of the reporting period.

In the following examples the parts of the reports being discussed will be shown, to reduce complexity and confusion the rest of the TAF report has been omitted.

Airport details and report duration

A TAF will always start with several key pieces of information, the airport ICAO code the report is generated for, the time the report was published and the duration of the weather report.

The weather

Wind

The first weather component reported in a TAF is the wind. This is given as a set of 5 numbers for winds up to 99kts and 6 numbers for winds over 99kts (not that many pilots are going to be worried about flying with winds over 99kts!). The wind reported will be the forecast steady wind, however, a gust factor can also be included in the report by the addition of the letter G after the first 5 characters with the gusting wind speed after. It is worth noting that the gust fact speed is in the middle of the forecast gust range and can be up to 10kts lower and higher than the reported value.

If the wind is varying with no defined direction VRB will be reported, followed by the wind speed, which can include a gust factor. If calm winds are forecast this will be reported as all zeros implying no direction and no strength.

Visibility

Visibility is the next component in a TAF report, this is given in meters or statute miles (or fractions thereof if being reported in the USA). The phrase often heard in general aviation terms is ‘all the nines’, this implies a visibility of 9999 being reported which is greater than 10km, or basically unlimited visibility ideal for VFR navigation.

Weather phenomena that have a direct impact on visibility can also appear after the visual range. These are generally used when the visibility is low and provide pilots greater understanding as to what is causing the reduction, examples can be fog or mist.

Weather

Precipitation or any other weather phenomena that will be present during the time period of the report will be detailed using the weather codes listed below. Some of these codes can be combined, for example RA would imply moderate rain, however, it can be combined with TSRA which changes the forecast into thunderstorms and moderate rain. Some of the codes can also be preceded with a plus + or minus - symbol which changes their intensity classification to heavy or light, +RA means heavy rain, -RA would be light rain. Finally if the weather is expected to be near the airport, but not directly impacting the airport, the code VC may be used, this denotes the weather is in the vicinity.

Types of Precipitation

RA	Rain
DZ	Drizzle
GR	Hail
SN	Snow
SG	Snow Grains
IC	Ice Crystals
PL	Ice Pellets
GS	Small Hail or Snow Pellets

Phenomena that affect Visibility

FG	Fog (Visibility less than 1000 meters)
BR	Mist (Visibility greater than 1000 meters)
HZ	Haze
FU	Smoke
SA	Sand
SS	Sand Storm
DU	Dust
DS	Dust Storm
VA	Volcanic Ash
PY	Spray

Significant Threat Clouds

TS	Thunderstorm
FC	Funnel Cloud
+FC	Tornado or Waterspout

Cloud

Cloud bases are reported to the nearest hundred feet and a TAF report can contain multiple layers of cloud. The amount of cloud is reported as either FEW, SCT, BKN or OVC referring to few, scattered, broken or overcast depending on the amount of the sky that is being obscured by the cloud layer.

FEW	Few 0/8th to 2/8th coverage
SCT	Scattered 3/8th to 4/8th coverage
BKN	Broken 5/8th to 7/8th coverage
OVC	Overcast 8/8th or complete coverage

The base of these cloud layers will then be reported with a 3 digit number reported in hundreds of feet above the airport. Finally the cloud type may be listed if it's either cumulonimbus or tower cumulus, as these types of clouds pose significant threats to aviation. In these instances the codes CB or TCU will be appended after the cloud height.

If the visibility is basically unlimited (9999 or greater than 10km) and the lowest cloud base is greater than 5000ft with no other weather phenomena, then a single phrase is used to reduce the complexity of the report. CAVOK or Cloud And Visibility OK is reported in place of the visibility and cloud, this does not mean however there are no clouds, you can still have a completely overcast sky above 5000ft.

Now that you understand how to read the weather that is going to occur within a TAF report, the next step is to understand when and how that weather may change.

Changes to the weather in the forecast time period

With each change in the weather, the changes will be listed as either a probable change, a temporary change, a becoming change or a from change, so what do these actually mean?

PROB30 & PROB40

Probable changes to the weather forecast are listed as either a PROB30 or PROB40, the definition being probability 30% or probability 40% of the change occurring. However, in simpler terms most pilots will assume a PROB30 is unlikely to happen and a PROB40 is likely to happen.

TEMPO

The word TEMPO in a TAF report will always be followed by 2 sets of four numbers, these represent the hours between which the temporary condition will occur. During this time period the conditions listed under the TEMPO are likely to occur for up to half the time, but may not be as a single occurrence depending on the duration of the TEMPO time period. It is also commmon to have a PROB30 or PROB40 prior to the TEMPO denoting the likihood of the temporary conditions.

BECMG

Becoming is a fundamental change to the TAF reported weather, this means from this point on the weather following the BECMG tag will be present until the end of the reporting period or until a further change is made via a becoming tag or a from tag. BECMG tags will be followed by 2 sets of four numbers, these are the hours during which this change will take place and is normally a gradual change over several hours.

FM

As with a BECMG a FR is a fundamental change to the underlying weather for the duration of the report. However, unlike a BECMG a FR, or from, is a defined time where a more rapid change in the conditions can be expected. The FR tag will be followed by six digits, this is the date and exact hour at which the change is likely to start occurring, with the total duration of the change often taking less than an hour.

Regional Variations

Although TAF reports are written to an international standard, regional variations to the units used do exist.

The most noticeable difference is the visual range, in Europe and many other parts of the world the visual range is presented in meters. However, in the United States the visual range is presented in statute miles, or fractions of. To allow easy identification of this difference, the visual range if being reported in meters will be presented as a number, whereas if the visual range is being reported in statute miles the number will be followed by the letters SM

TAF Glossary

The following is a list of abbreviations you may find in a TAF that have not already been covered in the guide above

AMD	Forecast has been amended since originally published
AUTO	Automatically produced without human interaction
BC	Patches (used with phenomena such as fog)
BE	Between (used to indicate a time range for a specific weather condition)
BL	Blowing (used with phenomena such as snow)
KT	Knots
M	The number following an M is a minus value
NOSIG	No significant changes expected in weather conditions
NSW	No significant weather
RMK	Remarks (usually followed by free flowing text)
RWY	Runway
SKC	Sky Clear
SPECI	Special Report
TN	Minimum forecast temperature
TX	Maximum forecast temperature
VC	In the vicinity of (used to indicate weather conditions in the vicinity of the aerodrome)
VRB	Variable (used with reference to the wind direction)
VV	Vertical visibility (often shown at VV/// denoting sky obscured or no vertical visibility)
WS	Wind shear (a sudden change in wind speed and or direction over a short distance)

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