METAR, TAF and SPECI are types of aviation meteorology reports to convey current weather conditions at airports and airfields:

• METAR: 'Meteorological Aerodrome Report'. It is a standard format for reporting weather information and is typically issued every hour. METAR provides a comprehensive summary of the weather conditions at an airport or aerodrome at a specific time.
• SPECI: 'Special Weather Report'. This type of report is issued to highlight significant changes in weather conditions since the last METAR. SPECI reports are generated whenever there are specific weather phenomena that exceed predetermined thresholds, such as significant drops in visibility, changes in wind direction or speed, and other important weather changes.

The ICAO code of the airport or weather station. Typically, this is a 4-letter code, e.g. "EGLL" is London Heathrow Airport.

The date and time in METAR and TAF reports are presented in a specific format that indicates when the observation was made or the forecast is valid. This format is crucial for determining the timeliness and relevance of the information.

Format in METAR:

• The date and time are indicated by the first four digits after the station identifier.
• The format is DDHH, where DD is the day of the month and HH is the hour of the day in UTC time.
• For example, a METAR report with '2518' indicates an observation made on the 25th day of the month at 1800 UTC.

Format in TAF:

• The TAF report includes the date and time of issuance as well as the validity period of the forecast.
• The issuance time is indicated in the same format as METAR, DDHH.
• The validity period is denoted by two sets of DDHH groups, indicating the start and end of the forecast period.
• For example, a TAF report with '2518/2618' means it is valid from the 25th at 1800 UTC to the 26th at 1800 UTC.

The terms COR, AUTO, and NIL have specific meanings in the context of METAR and TAF reports, indicating the nature of the report generation and its status.

• COR: 'Corrected'. It is used when a previously issued report has been corrected for errors or significant changes. A report with 'COR' suggests it is an updated version of an earlier report.
• AUTO: Indicates that the report was generated automatically without human intervention. Automated reports are common and indicate routine weather observations made by automated systems.
• NIL: Used to indicate that a usually expected report, such as a TAF, is not available. 'NIL' suggests that the report is missing or not issued under certain circumstances.

The wind information in METAR and TAF reports provides critical data regarding wind direction, speed, and gusts.

• The wind information is typically the second group of data in a METAR report, following the station identifier.
• It consists of five or more digits, sometimes followed by additional information for gusts and variability.
• The format is DDDSSKT, where DDD is the direction the wind is coming from in degrees true north, and SS is the speed in knots.
• If there are gusts, the letter 'G' follows the speed, along with the maximum gust speed (e.g., 22015G25KT indicates wind from 220 degrees at 15 knots, gusting up to 25 knots).
• When the wind direction is variable, it is indicated by 'VRB' instead of a specific direction (e.g., VRB10KT).
• In TAF reports, wind information may change over different forecast periods, and each period's wind data is presented in the same format as in METAR.

Special Cases:

• When the wind speed is less than 2 knots, it is reported as calm and denoted by '00000KT'.
• Wind direction variability, when it exists, is indicated at the end of the wind group with two directions between which the wind varies (e.g., 180V240).

Visibility information is a key component of METAR and TAF reports, indicating the distance over which objects can be clearly seen.

• Visibility is typically reported in meters or kilometers.
• In METAR reports, it appears after the wind information and is represented by a four-digit number indicating visibility in meters. For example, '1500' signifies 1500 meters of visibility.
• In cases where visibility exceeds 10 kilometers, it may be reported as '9999', indicating excellent visibility conditions.
• TAF reports follow a similar format, providing forecasted visibility values for specified time periods.

CAVOK (Ceiling and Visibility OK):

• Extended Visibility: Visibility must exceed 10 kilometers, indicating clear and unobstructed views over a large distance.
• Cloud Cover: There should be no cloud presence below 5,000 feet or below the minimum sector altitude, whichever is higher.
• Absence of Significant Cloud Types: Particularly noteworthy cloud types such as Cumulonimbus (CB) or Towering Cumulus (TCU) must not be present.
• No Significant Weather Phenomena: Conditions like rain, snow, thunderstorms, or any other weather phenomena that could impact visibility or flight safety should be absent.

The term 'CAVOK' effectively communicates ideal weather conditions with respect to visibility, cloud cover, and the absence of significant weather phenomena, important for aviation activities.

Weather phenomena in METAR/TAF reports are described using a combination of codes that denote their intensity, characteristic, and type. Understanding these codes is crucial for interpreting the weather conditions.

Cloud information in METAR and TAF reports includes details about cloud type, coverage, and altitude.

• Cloud coverage is reported in oktas, indicating the proportion of the sky covered by clouds.
• Each cloud layer is described by its type, coverage, and base altitude above ground level in hundreds of feet.

Atmospheric pressure provides current barometric pressure settings at the location of the weather station.

• Atmospheric pressure is typically reported in hectopascals (hPa), also equivalent to millibars (mb).
• In some countries, including the United States, pressure may be reported in inches of mercury (inHg).
• The format in a METAR report is 'QNH' followed by four digits representing the pressure in hectopascals. For example, 'QNH1013' indicates a pressure of 1013 hPa.
• In the case of reports using inches of mercury, the format is 'A' followed by four digits, where the first two digits represent the whole inches and the last two the decimal part. For example, 'A2992' represents 29.92 inHg.

Temperature and dew point information in METAR and TAF reports are crucial for understanding the current and forecasted atmospheric conditions.

• The temperature and dew point are reported in degrees Celsius.
• In METAR reports, they appear after wind and visibility data, formatted as 'TT/DD', where TT is the temperature and DD is the dew point.
• Both values can have a 'M' prefix to indicate minus, representing temperatures below zero. For example, 'M02/M05' means the temperature is -2°C with a dew point of -5°C.
• In TAF reports, the temperature forecast is often included, especially when significant changes are expected. It follows the same format as in METAR.

Trend information in METAR and TAF reports provides essential insights into expected short-term weather changes. These trends help in anticipating shifts in weather elements crucial for aviation operations.

• BECMG: Indicates a gradual change in weather conditions that is expected to occur. For example, 'BECMG 1500' suggests changes starting from 1500 UTC.
• TEMPO: Used for temporary fluctuations in weather, typically lasting less than an hour. E.g., 'TEMPO BKN007' indicates a temporary presence of broken clouds at 700 feet.
• PROB30: Shows a 30% probability of the specified change occurring. Changes with a lower probability are not typically mentioned. Example: 'PROB30 TEMPO 0200' indicates a 30% chance of temporary visibility reduction to 200 meters.
• PROB40: Indicates a 40% probability of the specified weather change. Higher probability changes are mentioned without the PROB indicator.
• FM (From): Specifies the time from which a change is expected. For instance, 'FM1200 6000' means visibility is forecasted to be 6000 meters starting from 1200 UTC.
• TL (To) and AT (At): These indicate the time until which, or at which, a certain weather condition is expected. 'TL1300' suggests the condition will last until 1300 UTC, while 'AT1500' indicates the condition is expected precisely at 1500 UTC.