What is METAR and how to decode it?

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What is METAR and how to decode it?

What is METAR?

In Aviation, weather plays a very important role in day to day operations. In fact, a pilot needs to check whether several times during the different phases of flight. Before the airplane is ready to leave the gate, he/she always checks the weather at the arriving and departing aerodrome in the pre-flight briefing. If they feel that the weather at and around the airport is not suitable for flying, they might decide not to operate the flight or even divert to their alternate aerodrome.
In aviation, the weather information of an aerodrome is broadcasted in a report called METAR (Meteorological Aerodrome Report). They are published in a fixed format which is common all over the world.

METAR is a string of abbreviations which denote various weather conditions around the aerodrome. You can learn all the METAR Abbreviations by referring to this document. With the help of this document, let us take an example of METAR of Chennai International Airport and decode it!

Eg: VOMM 131530Z 16006KT 4000 HZ SCT020 FEW025TCU BKN100 30/23 Q1009 NOSIG


Abbreviation
Indicates
Meaning
VOMM
ICAO Code
Chennai Intl Airport
131530Z
Date and Time
13 - Day of the month. 1530Z - Time of observation.
16006KT
Wind Direction
Wind direction is from 160 deg at 06 knots (denoted in Meters per second in Metric)
4000
Visibility
Prevailing Visibility at the airport is 4 km or 4000m
HZ
Weather Condition
Haze
SCT020
Cloud condition
Scattered clouds present at 2000ft
FEW025TCU
Cloud Condition
Few clouds present at 2500ft in towering cumulus form
BKN100
Cloud Condition
Broken clouds present at 10,000ft above aerodrome level
30/23
Temperature and Dew Point
The temperature at the airport is 30 deg. Dewpoint is 23 deg
Q1009
Sea Level Pressure / Barometer setting QNH
1009 hectopascals ( denoted in inch of Mercury - inHg in FAA)
NOSIG
Remark
No significant change in weather for the next 2 hours

SPECI Reports: 

A SPECI is a special report of meteorological weather conditions which is issued when one or more conditions match the specified criteria significant to aviation operations. It is also used to identify reports which are recorded ten minutes after an improvement to the SPECI conditions.

SPECI Example: SPECI VMMC 242341Z 10010KT 3500N VCSH FEW010 SCT018 BKN070 28/26 Q1004 TEMPO 5000 SHRA

SPECI Criteria : 



Element
Criteria
Wind Direction
Direction changed by 30 deg when the average speed is 20 KT or more.
Wind Speed
Speed changed by 10 KT when the average speed is 30 KT or more
Wind Gusts
Gust speed exceeds by 10 KT than the last reported speed
Visibility
When prevailing visibility is below the minimum operable limit of the aerodrome
Weather Phenomenon
When any of the weather phenomena such as thunderstorms, hail storms, sandstorms, dust storms, etc are reported in high intensity.
Cloud
When cloud cover (broken or overcast) is below the minimum operable limit of the aerodrome.
Temperature
When temperature increases or decreases by 5 deg since the last reported temperature
Pressure
When pressure increases or decreases by 2 hPa since the last reported pressure
Remarks
If the pilots repeatedly  report wind shear or other such weather phenomena

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