Geopotential Height Calculator for use in Meteorology and Aviation

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The advantage of using geopotential height/elevation instead of geometric height above mean sea level (AMSL) is that the geopotential height is already compensated for gravitational force variations with latitude and altitude/elevation.

METEOROLOGY: AMSL geopotential height is used to record the height of standard isobaric pressures (usually 500, 700, 850, 900, 925 mbar) at weather station locations or weather balloon positions. These data points are sewn together to plot standard isobaric surfaces (again 500, 700, 850, 900, 925 mbar) on plots called “Geopotential Height” of isobaric surfaces, which enable forecasters to easily identify low and high-pressure fronts and depressions to forecast the weather accurately.

AVIATION: Airplanes generally use geopotential altitude, not geometric. It allows pressure altitude calculations to use a fixed gravitational constant of 9.80616 m/s² and not worry about its variation with altitude and latitude when traveling long distances. It is based on the precise measurement of pressure by the airplane as it flies along a constant atmospheric pressure path, called a Flight Level (FL). Throughout each flight, its geometric (GPS) altitude will vary despite maintaining a constant flight level (pressure level). A flight level designated FL310 corresponds to flight at 31,000 feet and 314.84911 mbar standard isobaric pressure altitude.

FACTS:

  • Geopotential altitude is geopotential height expressed as height above mean sea level.

  • Geopotential units are called “geopotential meters” (gpm) or feet.

  • Geopotential height increases slower than geometric height as altitude increases since gravitational force weakens with altitude.

  • Geopotential height increases faster as one gets further north or south of the equator since the centrifugal force due to the earth’s rotation decreases.

  • Most high-altitude weather stations like the MeteoHelix IoT Pro report geopotential height of the closest standard isobaric surface pressure level instead of reporting the equivalent mean sea level pressure.

  • Generally, the gravitational constant is fixed at 9.80616 m/s². In actuality, the gravitational constant varies from approximately 9.75 to 9.832 m/s² depending where on earth’s surface you are located. It also decreases with altitude.

  • Pressure Altitude and Flight Level differ from geopotential altitude since real atmospheric conditions will always vary slightly from the International Standard Atmosphere (ISA) model as used by ICAO for aviation purposes. Not to mention the ISA atmospheric model assumes air humidity is zero.

Most high-altitude weather stations like this MeteoHelix IoT Pro report geopotential height of the closest standard isobaric surface pressure level instead of reporting equivalent mean sea level pressure.

500 mb geopotential height forecast by the United States numerical weather prediction model NAM. Also an example of an Omega Block. (Source: NWS, which is a branch of NOAA, a US government agency, Public domain, via Wikimedia Commons)

500 mb geopotential height forecast by the United States numerical weather prediction model NAM. Also an example of an Omega Block. (Source: NWS, which is a branch of NOAA, a US government agency, Public domain, via Wikimedia Commons)