How to Read a METAR: A Practical Guide to Aviation Weather Reports

Welcome to the first in a series of guides in which I hope to make operational weather information more accessible to the people who depend upon it.

During my years as a Senior Meteorology Officer with the Indian Air Force, providing weather support for flying operations, I observed that the METAR was almost always the first piece of information any crew requested. A forecast describes what is expected to happen; a METAR describes what is actually happening at that aerodrome, at that moment, as observed and encoded by a trained observer. For anyone making a decision within the next hour, that distinction is a significant one.

The format can appear daunting at first sight, but it is entirely logical. Every METAR follows the same fixed sequence of groups, in the same order, at every aerodrome in the world. Once that sequence becomes familiar, the report is straightforward to read.

Let us work through a representative example from Delhi:

METAR VIDP 191200Z 09008KT 4000 HZ SCT020 FEW030TCU 34/24 Q1004 NOSIG

Report type and station

METAR indicates a routine observation, issued at scheduled intervals, generally every half hour or hour depending upon the aerodrome. SPECI indicates a special observation, issued outside the normal schedule because conditions have changed significantly since the previous report. A SPECI therefore deserves particular attention, as it is an indication that something has altered materially.

VIDP is the ICAO location indicator for Indira Gandhi International Airport, Delhi. Indian indicators begin with the letter V: VABB for Mumbai, VOMM for Chennai, VECC for Kolkata, and VOBL for Bengaluru.

Date and time

191200Z denotes the 19th day of the current month at 1200 hours, with Z signifying Zulu time, or Coordinated Universal Time.

Wind

09008KT indicates wind from 090 degrees at 8 knots. The direction is given in degrees true, and it refers to the direction from which the wind is blowing.

Several variations are commonly encountered:

• VRB03KT — variable in direction at 3 knots, typical of light and calm conditions
• 27015G28KT — from 270 degrees at 15 knots, gusting to 28 knots
• 00000KT — calm conditions
• 09008KT 050V130 — from 090 degrees at 8 knots, direction varying between 050 and 130

Where gusts are reported, I would recommend that crosswind and performance calculations always be made against the gust value rather than the mean, as the aircraft will experience the peak.

Visibility

4000 indicates a visibility of 4,000 metres. Outside North America, visibility is reported in metres using four digits.

The value 9999 is worth committing to memory, as it signifies visibility of 10 kilometres or greater rather than a literal 9,999 metres. At the opposite extreme, 0000 indicates visibility of less than 50 metres.

Present weather

HZ denotes haze. This group uses a compact vocabulary of two-letter codes which combine to describe conditions precisely:

Intensity is indicated by a prefix, so that -RA is light rain, RA moderate, and +RA heavy. A group such as +TSRA, indicating a heavy thunderstorm with rain, carries considerable operational significance and should be treated accordingly.

The distinction between mist and fog is determined solely by visibility: below 1,000 metres the phenomenon is reported as fog, and at or above 1,000 metres as mist. The physical process is identical in both cases, though the operational consequences differ substantially.

Cloud

SCT020 FEW030TCU comprises two cloud groups, and this is the portion of the report most frequently misinterpreted.

The three-letter abbreviation describes the amount of sky covered, measured in oktas, or eighths:

The three digits which follow give the height of the cloud base in hundreds of feet above aerodrome level, not above mean sea level. At aerodromes situated at elevation, such as Leh, this distinction becomes particularly important.

Two further points deserve emphasis. First, a ceiling is defined only by a BKN or OVC layer. A sky containing several FEW and SCT layers, however cluttered it may appear, has no ceiling at all.

Temperature and dewpoint

34/24 indicates a temperature of 34 °C and a dewpoint of 24 °C. Negative values are prefixed with M, so that M02/M05 represents minus two over minus five.

The difference between these two figures, known as the dewpoint depression, is among the most useful quick diagnostics available in the report. A wide separation indicates dry air, while a narrow separation indicates air approaching saturation. When temperature and dewpoint converge to within two or three degrees during the evening, with clearing skies and decreasing wind, the formation of radiation fog becomes a genuine possibility. This is precisely the sequence which produces the well-known winter fog at Delhi, Lucknow, Amritsar and Varanasi each year.

Pressure and trend

Q1004 gives the QNH, or altimeter setting, in hectopascals. Setting 1004 on the subscale will cause the altimeter to indicate height above mean sea level. Reports originating in the United States use the prefix A with inches of mercury instead.

NOSIG indicates that no significant change is expected within the following two hours. Alternatively, BECMG indicates a permanent change is expected, while TEMPO indicates temporary fluctuations, each of less than one hour in duration.

Interpreting the report as a whole

Taken together, the example describes the following conditions:

Delhi, on the 19th at 1730 hours Indian Standard Time. Wind easterly at 8 knots. Visibility 4 kilometres in haze. Scattered cloud at 2,000 feet, with a few towering cumulus at 3,000 feet. Temperature 34 °C, dewpoint 24 °C. QNH 1004 hectopascals. No significant change expected.

The decoding, however, is only the first half of the task. The interpretation is the part to which experience contributes. These are hot, humid and hazy conditions, with towering cumulus already developing at low level during the afternoon. Towering cumulus represents the stage of development immediately preceding cumulonimbus. Although the trend group indicates no significant change, I would regard this as an aerodrome from which I would wish to obtain a fresh observation before committing, rather than one whose conditions I would assume to hold for the next two hours.

I hope this guide proves useful. Decoding a METAR is a mechanical skill which most people acquire within a week of regular practice. Interpreting one — recognising that a single two-letter suffix may outweigh every other element in the line, and that a trend group represents an expectation rather than a guarantee — develops over considerably longer, and it is that judgement which ultimately supports safe and effective decision-making.

Live METAR data for Indian aerodromes, decoded automatically, is available on our Live Weather Dashboard.