Weather Theory

Sea level 29.92 "Hg = 1,013.2 mb (hPa) = 14.7 lb/in2 12-5 yrucrem fo thgieH )mm 067( "29.92 the linkage mechanism of an aneroid barometer, it is not as At sea level in a standard accurate as a mercurial barometer. atmosphere, the weight of the atmosphere To provide a common reference, the International Standard (14.7 lb/in2) supports a column of mercury Atmosphere (ISA) has been established. These standard 29.92 inches high. conditions are the basis for certain flight instruments and most aircraft performance data. Standard sea level pressure e ur is defined as 29.92 "Hg and a standard temperature of 59 °F s e s (15 °C). Atmospheric pressure is also reported in millibars pr c (mb), with 1 "Hg equal to approximately 34 mb. Standard sea eri level pressure is 1,013.2 mb. Typical mb pressure readings h p s range from 950.0 to 1,040.0 mb. Surface charts, high and low o m At 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. Figure 12-6. Although mercurial barometers are no longer used in the U. S., they are still a good historical reference for where the By tracking barometric pressure trends across a large area, altimeter setting came from (inches of mercury). weather forecasters can more accurately predict movement of pressure systems and the associated weather. For example, An aneroid barometer is the standard instrument used tracking a pattern of rising pressure at a single weather station to measure pressure; it is easier to read and transport. generally indicates the approach of fair weather. Conversely, [Figure 12-7] The aneroid barometer contains a closed vessel decreasing or rapidly falling pressure usually indicates called an aneroid cell that contracts or expands with changes approaching bad weather and, possibly, severe storms. in pressure. The aneroid cell attaches to a pressure indicator with a mechanical linkage to provide pressure readings. The Altitude and Atmospheric Pressure pressure sensing part of an aircraft altimeter is essentially As altitude increases, atmospheric pressure decreases. On an aneroid barometer. It is important to note that due to 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 Higher being at a higher altitude and is referred to as density altitude. e ru As pressure decreases, density altitude increases and has a sse rp pronounced effect on aircraft performance. cire h p so Differences in air density caused by changes in temperature m tA result in a change in pressure. This, in turn, creates motion in the atmosphere, both vertically and horizontally, in the form Lower of currents and wind. The atmosphere is almost constantly in Sealed aneroid cell motion as it strives to reach equilibrium. These never-ending Sealed aneroid cell air movements set up chain reactions that cause a continuing Sealed aneroid cell variety in the weather. Figure 12-7. Aneroid barometer.