A METAR, or Meteorological Aerodrome Report, is a standardized, coded weather observation used globally to transmit current weather conditions for a specific location, typically an airport. This format allows pilots, meteorologists, and flight planners worldwide to quickly interpret the data regardless of their native language. Providing a snapshot of surface weather, the report is regularly updated, often hourly, making it a vital tool for aviation operations.
Decoding the Station Identifier and Time Group
The report begins with a four-letter ICAO (International Civil Aviation Organization) station identifier designating the location of the observation. These identifiers are globally unique; codes starting with ‘K’ are used for the contiguous United States, while ‘EGLL’ identifies London Heathrow Airport. The next group is the date and time (DDHHMMZ), formatted as six digits followed by the letter ‘Z’. The first two digits represent the day of the month, and the following four digits denote the time in a 24-hour clock format.
The ‘Z’ signifies “Zulu time,” or Coordinated Universal Time (UTC), synchronizing all reports globally. If conditions change significantly between regular hourly reports, a special, unscheduled report called a SPECI is issued. The group “AUTO” may also appear after the time, indicating the data was generated by an automated observing system without human supervision.
Interpreting Wind Speed and Direction
The wind group is a five-digit number followed by ‘KT’ (knots), detailing direction and speed. The first three digits indicate the direction the wind is blowing from, measured in degrees true north. The following two or three digits represent the wind speed, with ‘KT’ confirming the speed is measured in knots.
If the wind is calm (less than one knot), the group is reported as ‘00000KT’. When the direction is variable and the speed is less than 6 knots, the direction is replaced by ‘VRB’ (variable), such as ‘VRB05KT’. Strong wind conditions include the letter ‘G’ for “gusts,” where the maximum gust speed is reported immediately after the mean wind speed (e.g., ‘27015G25KT’). If the direction varies significantly (60 degrees or more) at speeds above 6 knots, the report includes the extreme directional variations separated by a ‘V’, such as ‘27015KT 240V300’.
Understanding Visibility, Weather Phenomena, and Sky Conditions
Visibility
The prevailing visibility indicates the greatest horizontal distance at which objects can be clearly seen. In North American reports, visibility is typically reported in Statute Miles (SM), often including fractions like ‘1/2SM’ or ’10SM’. If the visibility is less than one-quarter of a statute mile, it is encoded as ‘M1/4SM’.
Weather Phenomena
The weather phenomena group uses two-letter abbreviations, sometimes preceded by a qualifier for intensity or proximity. Intensity qualifiers include a minus sign (-) for light, a plus sign (+) for heavy, and no sign for moderate intensity. Descriptive qualifiers further refine the weather, such as ‘SH’ for showers, ‘TS’ for thunderstorm, or ‘FZ’ for freezing. Common abbreviations like ‘RA’ for rain, ‘SN’ for snow, and ‘FG’ for fog are combined with these qualifiers to form codes like ‘+SN’ (heavy snow) or ‘-DZ BR’ (light drizzle and mist).
Sky Conditions
The sky condition group is reported in layers, starting from the lowest altitude. Cloud coverage is categorized by the amount of the sky obscured using abbreviations: ‘FEW’ (few), ‘SCT’ (scattered), ‘BKN’ (broken), and ‘OVC’ (overcast). The height of the cloud base is reported in hundreds of feet Above Ground Level (AGL) using three digits; for example, ‘BKN025’ means a broken layer at 2,500 feet. If the sky is obscured by a surface-based phenomenon, the cloud group is replaced by ‘VV’ for Vertical Visibility. The digits following ‘VV’ indicate the height, in hundreds of feet, to which a person can see vertically into the obscuration.
Reading Temperature, Altimeter Setting, and Remarks
Temperature and Dew Point
The temperature and dew point are reported as a pair of two-digit numbers separated by a forward slash, measured in degrees Celsius. The first value is the air temperature and the second is the dew point. Temperatures below zero degrees Celsius are indicated by the letter ‘M’ (minus) preceding the value, such as ‘M02/M05’. A small difference between these two values indicates high humidity and a greater likelihood of fog or frost.
Altimeter Setting
The altimeter setting is reported using the letter ‘A’ followed by four digits. This value represents the barometric pressure in inches of mercury (inHg). The decimal point is assumed to be placed before the last two digits, so ‘A2992’ translates to 29.92 inches of mercury. This setting is used by pilots to calibrate their altimeters for accurate altitude readings.
Remarks (RMK)
The final section, Remarks, begins with the identifier ‘RMK’ and contains supplementary information primarily for meteorological use. This section may include details like the type of automated station (‘AO2’) or specific cloud types such as Cumulonimbus (CB). A common remark is the Sea Level Pressure (SLP), which provides meteorologists with a pressure value corrected for temperature.