Differences Between Barometric Pressure & Altimeter Settings
According to yourdictionary.com, a barometer measures "the pressure of the atmosphere" on the earth. In weather prediction, changes in the air pressure usually arrive in advance of, or concurrent with a change in wind, temperature and humidity. An altimeter "[measures] height above a given reference level" usually the altitude above the ground or sea level. The barometric pressure drops as altitude rises, and rises as altitude drops.
Other People Are Reading
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Function
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Meteorological barometers measure the high and low atmospheric systems that shift across the earth's surface. More accurate weather forecasts measure barometric changes (measured in inches or millimeters of mercury) by including wind, temperature and humidity in the calculations.
An aircraft's calibrated altimeters display the pressure at an altitude above sea level. They compute air pressure like a meteorological barometer, then convert it to an altitude reading.
The Facts
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Engineers usually measure pressure in pound per square inch (psi) where meteorologists measure atmospheric pressure in millibars. Aircraft altimeters convert pressure readings to show altitude above sea level in keeping with a typical pressure-altitude conversion ratio.
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Considerations
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The drawback with the typical altimeter is that it measures altitude based on the atmospheric pressure, which means that a drop in barometric pressure indicates an increase in altitude, and a rise in barometric would reflect a decrease in altitude. This persists even when the plane is stationary on the tarmac. If a storm with a low pressure passes through, the altimeter shows a rise in altitude. As the storm system moves out, the pressure rises, dropping the altitude.
Warnings
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Outside air temperature also affects altimeter readings. Temperatures that are warmer than they are normally, will cause the altimeter to give an altitude that is lower than the actual altitude; meaning the plane is higher than what the altimeter shows. Cold weather, on the other hand, signals an error in the opposite direction. The result is in an aircraft flying lower than what the altimeter shows. This is a critical error, which could result in a plane hitting the ground when the altimeter shows it should still be in the air.
Prevention/Solution
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The pilot needs to adjust the altimeter, correcting for the atmospheric conditions, in order to obtain an accurate altitude reading. Aircraft altimeters include a small display called a subscale. Its measurements are marked off in small increments linked to the barometric pressure. The subscale can be fine-tuned to adjust for changes in barometric pressure from one region to the next based on that station's weather report. The pilot will also receive altimeter settings from Air Traffic Control at every airport he passes while en route. This information is so essential that it may be transmitted numerous times within a brief period of time. The pilot should check his instruments every time a setting is relayed and read back his altimeter reading for verification from Air Traffic Controller.
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