Email:
John Woodside

TAF - Aerodrome Forecast

An aerodrome TAF is a forecast - it will give the predicted weather conditions expected at an aerodrome, usually for a 9 or 24 hour period. The standard 9 hour TAF is updated and reissued every three hours.

METAR - Aerodrome Actual Met Report

The METAR is a report giving the actual weather conditions at an aerodrome at the time of the report. METAR's are issued every 30 minutes during the opening hours of the aerodrome.

Identifier Components

1. Report Type - METAR
2. Location Indicator - ICAO four letter aerodrome code.
3. Time - The time of the observation in UTC (GMT) in hours and minutes followed by the letter Z (the universal indication that GMT time is being reported).

Example: METAR EHAM 1050Z

METAR Weather Components

1. Wind
2. Visibility
3. RVR
4. Weather
5. Cloud
6. Temp/Dew Point
7. QNH
8. Recent Weather
9. Wind Shear
10. Trend

1. Wind

Wind is given as direction in degrees true followed by the average wind speed.
Example:24015KT - indicates that the wind is blowing from the southwest at 15 kts..
Note: KMH (Kilometres per hour) or MPS (Metres per second) may also be found in this group e.g 34025KPH.

If the wind is gusting this will be shown by a further group of figures preceded by the letter G. Example:

24015G27KT

this indicates that the average wind is 240 at 15 kts but gusts to 27 kts are recorded.

If the wind is calm then 00000KT will be shown.

A variable wind direction will be shown by VRB e.g. VRB02KT would indicate that the wind was only 2 kts but would not be blowing from any fixed direction.

If the wind is greater than 3 kt and the direction is varying by 60 degrees or more then the actual values must be recorded. Example:

31015G27KT 280V360

shows that the wind is blowing from the northwest at 15 kts with gusts to 27 kts but that the wind direction is varying from 280 to 360 degrees. Not very nice...

2. Visibility

Visibility is recorded as a four figure group in metres (2000 metres is roughly 1 nm).
If 0000 is shown it would indicate that the visibility is less than 50 metres.
If 9999 is shown it indicates that the visibility is 10 km or more.

If local conditions vary greatly then two groups may be displayed showing the visibility variation in different sectors e.g.

1000NW 6000S

- showing that the visibility to the northwest of the aerodrome is only 1000 metres but is 6 km to the south.

3. RVR (Runway Visual Range)

Runway Visual Range is an indication of the real visibility as measured down the runway either electronically or manually. RVR is taken when the Met visibility drops below 1500 metres and it will therefore only be shown occasionally in METAR reports. RVR visibility will always be prefixed by the letter R followed by the runway for which the value has been taken. Example:

R24/1200 - RVR for runway 24 is 1200 metres

It may be of interest to you to know that the RVR value recorded is significant to pilot operations. If the RVR value is below that published for the approach procedure then the aircraft CANNOT MAKE AN APPROACH. The aircraft must either remain in the holding pattern until the weather improves or it must divert.

4. Weather

Weather is identified by one or more two letter groups. The full list is not very long so I shall include them all here - even the more obscure phenomena <g>...

5. Cloud

Usually this is a six figure group and one that most of you will already recognise. The group consists of three letters that describe the cloud cover followed by three figures for cloud height ABOVE AERODROME LEVEL.

Cloud amount is given as;

Cloud height is given by the next three figures which show the altitude in hundreds of feet. i.e. 040 is 4000 ft, 004 is 400 feet, 200 is 20,000 ft. Examples:

SCT020 - Scattered at 2000 ft.
BKN005 - Broken cloud at 500 feet.
OVC250 - Overcast at 25,000 feet.

A METAR may contain several cloud layers so you may get: SCT025 BKN070 BKN120 -

showing scattered cloud at 2500 ft, broken cloud at 7000 feet and again at 12,000 feet.

SKC - (Sky Clear) will be used if no cloud layers are observed.
CAVOK - Used if no cloud exists below 5000 ft AND the visibility is greater than 10 km AND there is no fog, precipitation or snow.
CB - Used to emphasise Cumulonimbus formation - BKN015CB
TCU - Used to signify Towering CB formation.
NSC - Signifies that no significant cloud exists above 5000ft.

In conditions of fog when the cloud cover cannot be seen then the vertical visibility will be reported using VV as the code.
e.g. VV003 = vertical visibility 300 feet.
If the fog is so bad that no measurement can be taken then you may possibly see VV/// in a METAR.

6. Temperature and Dew Point

Temperature and Dew Point are both measured in Centigrade. A minus value is preceded by the letter M. Examples:

25/12 = Temp 25°C, Dew Point 12°C, or
00/M02= Temp 0°C, Dew Point -2°C.

7. QNH (Barometric Pressure)

QNH is rounded down to the next whole millibar and reported as a four figure group preceded by Q. If the value is less than 1000mbs then the first digit will be 0.

Examples;

Q0996, Q1030.

To explain this further may I just add that if you set the QNH value on your altimeter then the height displayed on the instrument will be your height above sea level. If you are on the ramp then your altimeter should read airport elevation.

One other side factor of pressure is on aircraft performance. If QNH is low then an aircraft will perform much more badly (less lift, slow climb, poor engine performance) because it is operating in less dense air.

8. Recent Weather

This may be included if appropriate using the weather codes above, prefixed with RE (for recent).
Example RERA for recent rain..

9. Wind Shear

Again this will only be included if appropriate. The code WS is used followed by the runway affected.
Example:

WS LDG RWY28L or WS TKOF RWY08

10. Trend

This is only available at selected airfields. It is intended to indicate significant changes of weather in the two hours after the observation is made. If the change is temporary then TEMPO is used followed by the predicted conditions. If the change is permanent them BECMG (Becoming) will be used. Example:

TEMPO 3000 SHRA = temporary visibility 3000 metres with rain showers.
BECMG 33035KT = becoming 35 kts of wind from 330.
NOSIG = no significant changes in weather.