The Meteorological Aviation Routine Weather Report (METAR) is the international standard format for providing current weather conditions at airports and airfields. This report uses coded data that pilots and forecasters use to assess flying conditions. Among the most fundamental pieces of information is the barometric pressure, which is significant for calculating accurate altitude in flight operations.
Understanding the Altimeter Setting (QNH)
The specific pressure value reported in a METAR is the Altimeter Setting, often referred to by the Q-code designation QNH. This value is derived by taking the atmospheric pressure measured at the station and mathematically correcting it to what it would be at mean sea level (MSL) at that moment, assuming standard atmospheric conditions. This correction is performed regardless of the actual elevation of the airport.
The purpose of the QNH setting is to provide a unified reference point for all aircraft. When a pilot sets the altimeter subscale to the reported QNH, the instrument displays the aircraft’s altitude above mean sea level (MSL). If the aircraft is resting on the runway, the altimeter will correctly display the published field elevation. This reliable altitude display is necessary for safe separation from terrain and other air traffic.
Identifying the Pressure Group in the METAR Code
The pressure information is positioned toward the end of the main body of the METAR report, following the temperature and dew point data. This placement is consistent globally, helping users quickly locate the data point regardless of the reporting country. The pressure group is always easily identifiable because it is preceded by a single letter that indicates the unit of measure.
In reports originating from North America, including the United States and Canada, the pressure value is preceded by the letter ‘A’, signifying inches of mercury. International reports typically use the letter ‘Q’ to denote the pressure group. The ‘Q’ indicates that the pressure is reported in hectopascals (hPa), also known as millibars.
Decoding the Pressure Value and Units
The interpretation of the four-digit number immediately following the prefix letter depends entirely on whether the report uses the ‘A’ or ‘Q’ designation. The two systems represent distinct units of pressure measurement, inches of mercury (inHg) and hectopascals (hPa). Correctly decoding this four-digit sequence is the most important step in utilizing the METAR pressure data.
When the group begins with the letter ‘A’, the four subsequent digits represent the pressure in inches of mercury, but with an implied decimal point. The decimal is placed between the second and third digits of the sequence. For example, a pressure group coded as A3001 must be read as 30.01 inches of mercury. A lower pressure value, such as A2950, translates to 29.50 inHg.
The international standard uses the ‘Q’ prefix, which represents pressure in hectopascals. For these reports, the four-digit number is the direct pressure value in hPa, with no implied decimal point to consider. A report showing Q1013, for instance, means the QNH is 1013 hectopascals. Similarly, a high-pressure system might be reported as Q1030, meaning 1030 hPa.
Both inches of mercury and hectopascals are conventional units for measuring atmospheric pressure. The use of one system over the other is purely a matter of regional convention, but both formats are standardized to ensure accurate decoding by pilots and meteorologists worldwide.
Why Accurate Pressure Reading is Critical
The primary function of the METAR pressure reading is to ensure the safety of flight, particularly during takeoff, landing, and flight at lower altitudes. If the reported QNH is incorrectly set into an aircraft’s altimeter, the instrument will display an altitude that differs from the aircraft’s true altitude above mean sea level. This difference between indicated and actual altitude is known as altimeter error.
A common phrase used in aviation to remember the consequence of this error is “High to low, look out below.” This means that if an aircraft flies from an area of high pressure into an area of lower pressure without updating the altimeter setting, the altimeter will indicate an altitude that is higher than the aircraft’s true position. The aircraft is therefore closer to the ground than the pilot believes, which creates a substantial risk of controlled flight toward terrain.
Even a small difference in pressure can lead to significant vertical errors, especially during an instrument approach. An error of just one-tenth of an inch of mercury (0.10 inHg) can translate to an altitude error of approximately 100 feet. This level of inaccuracy can negate the safety margins built into approach procedures. The correct and current altimeter setting from the METAR is thus fundamental safety data, directly affecting the aircraft’s vertical navigation and separation from the ground.