Altimeter Settings and Barometric Pressure
By Joan Whetzel
Barometric Pressure (air pressure) measures the weight of the atmosphere, which is basically how heavy the air feels at various altitudes. Barometric pressure not only changes with levels of altitude or depth but may vary (within a set range) when the weather changes. Pilots need to know the weather conditions (especially the barometric pressure) at their take off location, at their destination, and at all points in between because the barometric pressure will affect their altimeter readings.
Barometric Pressure and Altimeter Settings
Barometers measure the pressure of the atmosphere and are used to track changes in the weather. Barometer readings usually drop in advance of, or in sync with, a change in the wind, temperature and humidity.
Altimeters measure the height above a specified reference level - usually above the earth's surface or above sea level. These altimeter readings are based on the barometric pressure. In the cockpit of an airplane, the pilot will use either radar or an aneroid barometer to measure the altitude. The dial measure on the aneroid barometer indicates the plane's altitude in meters or in feet.
Altimeters vs. Barometers
Meteorologists use barometers to measure the shifts in atmospheric pressure as it moves across the earth's surface. These atmospheric changes are usually measured in millibars or in inches or millimeters of mercury and are calculated along with wind speeds, temperature readings, humidity levels, and cloud size and shape to achieve the most accurate weather forecast. Pilots and the aviation industry use calibrated altimeters in the airplane cockpits to display the air pressure. They measure the altitude above sea level rather than above the earth's surface because the earth's surface is so reliable or inconsistent - mountains, ravines, hills, flat lands, etcetera. This altitude above sea level reading is consistent with the mathematical model used by the International Standard Atmosphere (ISA). The air pressure is calculated in the same way as barometer readings for meteorological usage, and is then converted to an altimeter reading.
In engineering, air pressure is measured as psi, or pounds per square inch. Meteorologist, though measure air pressure in millibars or in inches or millimeters of mercury. Aircraft altimeters convert this measurement to show the altitude above sea level. Barometric pressure can change slightly, within a specified range, depending on the altitude and the local weather. At sea level, for instance, the barometric pressure can range from approximately 28 to 32 inches of mercury. At 2,000 feet above sea level that reading may vary between approximately 25 and 29 inches of mercury, while at 5,000 feet above sea level it may vary between approximately 23 and 27 inches of mercury.
The Disadvantages for Altimeter Usage
The main disadvantage for altimeters is that the altitude measurement is based on the barometric (atmospheric) pressure. What that means is that any drop or rise in the barometer readings could affect the altimeter reading. These changes could produce an inaccurate altimeter reading, making the planes altitude appear higher than it is (with a drop in local barometric pressure) or lower than it actually is (with a rise in barometric pressure). This is frequently the case even when the plane is standing still on the tarmac.
Another disadvantage is that the altimeter reading may also be affected by changes in air temperature. Warmer than average temperatures may cause the altimeter to indicate an altitude that is higher than the actual altitude. Colder temperatures may cause a lower than true altitude reading.
Adjustments must be made in the flight readings to account for these variables or the pilot could run into problems due to critical errors. If the altimeter is producing a reading that shows a false-high altitude (meaning the reading is higher than the plane's actual altitude) and the pilot cannot see the ground due to weather conditions or because he or she is flying at night, the plane could hit the ground when the altimeter is showing that the plane should still be air born.
The solution is for the pilot to be aware of the weather conditions and to make adjustments to the altimeter based on the those weather conditions. The altimeters in airplanes comes equipped with a smaller subscale display, which has small markings that are linked to the barometric pressure. The pilot simply needs to tweak the subscale to make the necessary changes in the local barometric pressure readings, which are obtained from each region's weather report station. The pilot also receives altimeter settings from the Air Traffic Control at each airport along his or her route. Air Traffic Control towers transmit this information repeatedly, at short intervals, so that pilots can verify their altimeter readings with each regional Air Traffic Control tower.
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