Measurement of Atmosphere Pressure

Atmospheric pressure historically was measured in inches of mercury ("Hg) by a mercurial barometer. [Figure 12-6] 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. Sea level 29.92 "Hg = 1,013.2 mb (hPa) = 14.7 lb/in2 yrucrem thgieH )mm 067( "29.92 At sea level in a standard atmosphere, the weight of the atmosphere (14.7 lb/in2) supports a column of mercury 29.92 inches high. in the U. S., they are still a good historical reference for where the altimeter setting came from (inches of mercury). An aneroid barometer is the standard instrument used to measure pressure; it is easier to read and transport. [Figure 12-7] 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 her ire Lower Sealed aneroidcell Sealed aneroidcell Sealed aneroidcell 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 "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. 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 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 "Hg, reports a sea level pressure reading "Hg. [Figure 12-8] 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 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.