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Measurement of Atmosphere Pressure

in Weather Theory

Atmospheric pressure is typically measured in inches of mercury (“Hg) by a mercurial barometer. [Figure 11-6] 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 a laboratory or weather observation station, is not easily transported, and difficult to read.

Figure 11-6. Mercurial barometer.

Figure 11-6. Mercurial barometer.

An aneroid barometer is an alternative to a mercurial barometer; it is easier to read and transport. [Figure 11-7] 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.

Figure 11-7. Aneroid barometer.

Figure 11-7. Aneroid 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 “Hg and a standard temperature of 59 °F (15 °C). Atmospheric pressure is also reported in millibars (mb), with 1 “Hg equal to approximately 34 mb. Standard sea level pressure is 1,013.2 mb. Typical mb pressure readings range from 950.0 to 1,040.0 mb. Constant pressure charts and hurricane pressure reports are written 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 converts its barometric pressure by adding approximately 1 “Hg for every 1,000 feet of elevation. For example, a station at 5,000 feet above sea level, with a reading of 24.92 “Hg, reports a sea level pressure reading of 29.92 “Hg. [Figure 11-8] Using common sea level pressure readings helps ensure aircraft altimeters are set correctly, based on the current pressure readings.

Figure 11-8. Station pressure is converted to and reported in sea level pressure.

Figure 11-8. Station pressure is converted to and reported in sea level pressure. -Click to Enlarge-

By tracking barometric pressure trends across a large area, weather forecasters can more accurately predict 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.

515G+mn0RuL._SX331_BO1,204,203,200_Learn more about aviation weather with Weather Flying by Robert Buck. Regarded as the bible of weather flying, this aviation classic not only continues to make complex weather concepts understandable for even the least experienced of flyers, but has now been updated to cover new advances in technology.



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