Pressure altimeters are barometers that indicate altitude. These instruments are based upon the assumption that air pressure decreases with increased altitude from some standard sea level air pressure at a fixed rate of decrease. Specifically, sea level pressure - taken at the reference altitude - is assumed equal to the standard 1013.25 millibar (mb) or (29.92 inches of mercury at 59 degrees Fahrenheit) and to have a vertical decrease in pressure equivalent to that specified by a model, usually the Standard Atmosphere. For practical purposes, the exponential decrease in pressure over the lowest 10,000 ft of the atmosphere can be approximated by a vertical pressure decrease of 0.30 inches of mercury for every 300 ft rise in altitude (or 10 mb for every 100 meter change). While this assumption is satisfactory for most purposes, several factors must be considered in order to obtain an accurate determination of altitude.
The altitude indicated by the pressure altimeter may not be identical to the true altitude because the actual conditions encountered typically will differ from the assumptions. Hence, an aircraft trying to maintain a constant altitude using a pressure altimeter is actually flying on an undulating constant pressure surface rather than on a flat constant altitude surface. This variation may become a serious problem when flying in mountainous areas or at low levels in poor visibility. Certain corrections can be made by the pilot to reduce the altimeter error.
The vertical temperature profile and average temperature of the layer at the moment may not be equivalent to that specified by the Standard Atmosphere. The indicated pressure altitude depends upon the average temperature (hence, the density) of the intervening air column between the altimeter and sea level.
If the average temperature of the intervening atmospheric layer were colder than the standard, the altimeter will read too high, and the pilot would be at a lower actual altitude than indicated on the pressure altimeter. For example, on a cold day, the indicated altitude may read 10,000 ft, but the actual altitude is 9500 ft because pressure decreases more rapidly in a cold air column. Conversely, if the mean temperature of the air layer were warmer than standard, the altimeter will read too low, and the pilot would be at an altitude higher than indicated. The indicated altitude of 10,000 ft may be actually 10,400 ft.
By measuring the free air temperature at flight level, the pilot can use an on-board flight computer to make an appropriate adjustment to the altimeter setting, based upon the departure of the existing air temperature from the hypothetical temperature of the Standard Atmosphere.
The mean sea level corrected pressure below the aircraft at the moment may not be equivalent to the standardized pressure of 1013.25 mb. When the surface air pressure is lower than the standard, the indicated altitude will be higher than the actual altitude.
A correction for this latter error can be applied by checking the "altimeter setting" reported at the airport before take-off. The altimeter setting represents the value of atmospheric pressure to which the scale of a pressure altimeter is set to indicate the true altitude of an aircraft at the airport elevation. The pilot would use this setting to adjust the movable scale on the aircraft's altimeter accordingly. This local altimeter setting incorporates the large scale changes in the observed air pressure at the airport at any given time. For flights below 18,000 ft, the pilot must routinely contact the local flight center to update altimeter settings along the flight path. Above 18,000 ft, a pressure altimeter would be set at the standard 29.92 inch altimeter setting.
Altimeter settings are available with other pilot information from the NWS's Aviation Weather Center.
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Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 1999, The American Meteorological Society.