The International Civil Aviation Organization (ICAO) has created the International Standard Atmosphere (ISA) standards and is based on average meteorological temperatures and pressures around the planet.
The ISA is a "model" of the atmosphere, designed to allow for standardised comparison of conditions on a given day. We can compare the actual air pressure or air density on any given day to the pressure or density in the ISA.
The ISA measures for temperature and pressure are:
- The temperature at Mean Sea Level (MSL) is averaged to be 15° C and cools with ascent through the atmosphere at a rate of 1.98° C per 1000 ft up to the tropopause. Referred to as the standard lapse rate, rounded off to 2° C per 1000 ft.
- The tropopause has an equal height around the planet of 36,090 ft Above Mean Sea Level (AMSL).
- Atmospheric pressure at Mean Sea Level (MSL) is averaged to be 1013.25 hPa and reduces with ascent through the atmosphere at a rate of approximately 1 hPa per 30 ft or 1 inch of hg per 1000 ft of altitude in the lower troposphere.
If the density at your altitude is the same as the density at 10,000 ft in the ISA we say you are at a Density altitude of 10,000 ft. If the pressure at your altitude is the same as the pressure at 5000 ft in the ISA we say you are at a pressure altitude of 5000 ft. Each of the preceding is true regardless of the actual altitude you are at.
The Q code is a standardized collection of three-letter message encodings, all starting with the letter "Q", initially developed for commercial radiotelegraph communication, and later adopted by other radio services, especially amateur radio. Although Q codes were created when radio used Morse code exclusively, they continued to be employed after the introduction of voice transmissions.
To avoid confusion, transmitter call signs have often been limited to restrict ones starting with "Q" or having an embedded three-letter Q sequence. The codes in the range QAA-QNZ are reserved for aeronautical use; QOA-QOZ for maritime use, and QRA-QUZ for all services.
Commonly used aviation Q codes:
- QNH Local Pressure at mean sea level (MSL).
- QNH Area Pressure at mean sea level (MSL).
- QNE Standard Pressure set to 1013.25 (hPa).
- QFE Aerodrome Pressure at the aerodrome elevation reference datum.