Accurate measurement of atmospheric pressure is essential for weather forecasting, altitude determination, and aircraft performance calculations. Pilots rely on pressure information to set flight instruments correctly and to evaluate how atmospheric conditions may affect flight operations.
Measurement of Atmospheric Pressure
Atmospheric pressure historically was measured in inches of mercury ("Hg) by a mercurial barometer. [Figure 1]
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| Figure 1. Although mercurial barometers are no longer used in the U. S., they are still a good historical reference for where the altimeter setting came from (inches of mercury) |
The barometer measures the height of a column of mercury inside a glass tube. A section of the mercury is exposed to the pressure of the atmosphere, which exerts a force on the mercury. An increase in pressure forces the mercury to rise inside the tube. When the pressure drops, mercury drains out of the tube decreasing the height of the column. This type of barometer is typically used in laboratories and weather observation stations, is not easily transported, and can be difficult to read.
An aneroid barometer is the standard instrument used to measure pressure; it is easier to read and transport. [Figure 2]
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| Figure 2. Aneroid barometer |
The aneroid barometer contains a closed vessel called an aneroid cell that contracts or expands with changes in pressure. The aneroid cell attaches to a pressure indicator with a mechanical linkage to provide pressure readings. The pressure sensing part of an aircraft altimeter is essentially an aneroid barometer. It is important to note that due to the linkage mechanism of an aneroid barometer, it is not as accurate as a mercurial barometer.
To provide a common reference, the International Standard Atmosphere (ISA) has been established. These standard conditions are the basis for certain flight instruments and most aircraft performance data. Standard sea level pressure is defined as 29.92 inHg, and standard sea level temperature is 59 °F (15 °C). Atmospheric pressure is also reported in millibars (mb), with 1 inHg equal to approximately 33.86 mb. Standard sea level pressure is 1,013.2 mb. Typical mb pressure readings range from 950.0 to 1,040.0 mb. Surface charts, high and low pressure centers, and hurricane data are reported using mb.
Since weather stations are located around the globe, all local barometric pressure readings are converted to a sea level pressure to provide a standard for records and reports. To achieve this, each station applies a correction factor based on its elevation and current atmospheric conditions to determine sea level pressure. For example, a station at 5,000 feet above sea level, with a reading of 24.92 inHg, reports a sea level pressure reading of 29.92 inHg. [Figure 3]
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| Figure 3. Station pressure is converted to and reported in sea level pressure |
Using common sea level pressure readings helps ensure aircraft altimeters are set correctly, based on the current pressure readings.
By tracking barometric pressure trends across a large area, weather forecasters can more accurately predict the movement of pressure systems and the associated weather. For example, tracking a pattern of rising pressure at a single weather station generally indicates the approach of fair weather. Conversely, decreasing or rapidly falling pressure usually indicates approaching bad weather and, possibly, severe storms.
Altitude and Atmospheric Pressure
As altitude increases, atmospheric pressure decreases. On average, with every 1,000 feet of increase in altitude, the atmospheric pressure decreases approximately 1 inHg. As pressure decreases, the air becomes less dense, producing a higher density altitude. As pressure decreases, density altitude increases and has a pronounced effect on aircraft performance.
Differences in air density caused by temperature variations can result in pressure differences within the atmosphere. This, in turn, creates motion in the atmosphere, both vertically and horizontally, in the form of currents and wind. The atmosphere is almost constantly in motion as it strives to reach equilibrium. These never-ending air movements set up chain reactions that cause a continuing variety in the weather.