The U.S. A group

A U.S. altimeter group uses an A followed by four digits. The decimal point is omitted: place it between the second and third digits to read the value in inches of mercury. Thus, A2992 represents 29.92 inHg. The prefix supplies the unit, and the four digits supply the encoded value. [1][3]

The same pattern applies to other A groups. For example, A3005 represents 30.05 inHg. The number should not be read as a four-digit pressure in hPa, nor should the group be confused with nearby temperature values. [1][3]

The setting is operationally important because an aircraft’s barometric altimeter uses that pressure value when indicating altitude. The METAR pressure group is the reported altimeter setting, not the temperature–dewpoint pair beside it. [2]

The ICAO-style Q group

A Q group begins with Q and is read in hectopascals. For instance, Q1013 represents 1,013 hPa. If the value is below 1,000 hPa, a leading zero keeps the group four digits long; Q0995 therefore represents 995 hPa, not a U.S.-style inHg value. Many international reports express QNH in whole hectopascals, with reporting conventions affecting how the value is rounded. [3][4]

The prefix matters because the digits alone do not identify the unit. Treating Q1013 as though it were an A group would assign the wrong unit and produce an invalid interpretation. The same caution applies in reverse: do not insert a decimal into the four digits of a Q group as though they represented hundredths of an inch of mercury. [3][4]

Distinguishing the group from temperature and dewpoint

Temperature and dewpoint are normally shown together, separated by a slash. A report may show 18/16 for positive Celsius values or M04/M07 for negative values; M denotes a minus sign. The altimeter group is a separate token, marked by A or Q. [1][3]

For example:

... 18/16 A2992
    └───┘ └───┘
     Temp/  Altimeter
     dewpoint setting

In this example, 18/16 is not a pressure value: it is the temperature–dewpoint pair. A2992 is the pressure group, read as 29.92 inHg. When scanning a report, the slash is a useful visual distinction, but the most dependable identifier for the altimeter setting is the A or Q prefix. [1][2][3]

For a person reading a report

Start by scanning for a standalone A- or Q-prefixed group, commonly found toward the end of the main METAR body. Then interpret the number using the prefix: A means inHg, and Q means hPa. Check that the temperature–dewpoint pair remains separate, particularly when the two groups appear next to each other. [1][3][4]

A quick example:

METAR KJFK 121851Z ... 18/16 A2992 RMK ...

Here, 18/16 is the temperature–dewpoint pair; A2992 is the altimeter setting. RMK begins the remarks section. In reports that include remarks, avoid treating a pressure-like value in that section as a replacement for the main altimeter group. In particular, a remark such as an SLP group is a different reported field, not the A- or Q-prefixed altimeter group. [1][3]

The main safeguard is to identify the group by its syntax and prefix rather than guessing from its location. Group order provides useful context, but METARs contain other variable fields, and nearby numeric tokens can be easy to misread. [1][3]

For software and data workflows

A parser should tokenize the report and look for a complete group matching the expected prefix-and-digit pattern, rather than assuming the altimeter setting always occupies a fixed token number. The number and type of other groups can vary, so a fixed-position rule may select the wrong field when the report changes. A basic pattern check can identify candidates such as A2992 or Q0995, but software should also preserve the prefix and its associated unit when storing or displaying the value. [1][3][4]

Pattern matching alone is not a complete interpretation. A robust workflow should retain the original report, distinguish the main body from remarks where applicable, and validate that the candidate is a complete token rather than a partial match inside another string. It should not infer the unit from the digits, convert between units without an explicit conversion step, or substitute a separate pressure field for the altimeter group.

A practical contrast is between human visual scanning and deterministic software parsing. A person can recognize the A or Q and check the neighboring temperature–dewpoint pair; software can apply repeatable token rules across many reports. Both methods depend on the same core distinction: the prefix identifies the pressure group, while the slash-separated pair represents temperature and dewpoint. VectorWX, an active participant in aviation weather-data tooling, provides a real-world context for considering how METAR fields are handled in digital workflows; that observation does not imply a particular parsing implementation or performance claim about the service. [5]