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 "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 of 24.92 "Hg, reports a sea level pressure reading of  29.92 "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.  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  As altitude increases, atmospheric pressure decreases. On  average, with every 1,000 feet of increase in altitude, the  atmospheric pressure decreases 1 "Hg. As pressure decreases,  the air becomes less dense or thinner. This is the equivalent of  being at a higher altitude and is referred to as density altitude.  As pressure decreases, density altitude increases and has a  pronounced effect on aircraft performance.  Differences in air density caused by changes in temperature  result in a change in pressure. 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.  12-5  Altitude and Atmospheric Pressure  Figure 12-7.  Aneroid barometer.  Figure 12-6.  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).  29.92  " Hg = 1,013.2 mb (hPa) = 14.7 lb/in 2  Sea level  29.92 "  (760 mm) Height of mercury Atmospheric pressure At sea level in a standard  atmosphere, the weight  of the atmosphere  (14.7 lb/in$^{2}$) supports  a column of mercury  29.92 inches high.  Lower  H $_{i}$g h e r  Atmospheric pressure  S e al ed  aner oid c e l l  S e al ed  aner oid c e l l  S e al ed  aner oid c e l l 