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STANDARD ATMOSPHERIC PRESSURE The '' and is defined as being precisely equal to 101.325 KPa . It is equivalent to 760 MmHg ( Torr ) or 29.92 InHg . One standard atmosphere is standard pressure used for pneumatic fluid power (ISO R554), and in the aerospace (ISO 2533) and petroleum (ISO 5024) industries. In 1982, the ) In the United States , compressed air flow is often measured in "standard cubic feet" per unit of time, where the "standard" means the equivalent quantity of air at standard temperature and pressure. However, this standard atmosphere is defined slightly differently: temperature = 68 °F (20 °C), air density = 0.0765 lb/ft³ (1.225 kg/m³), altitude = sea level, and relative humidity = 0%. In the air conditioning industry, the standard is often temperature = 32 °F (0 °C) instead. For natural gas, the petroleum industry uses a standard temperature of 60 °F (15.6 °C), pressure 14.73 psia. MEAN SEA LEVEL PRESSURE Mean sea level pressure (MSLP or QFF) is the pressure at sea level or (when measured at a given elevation on land) the station pressure reduced to sea level assuming an isothermal layer at the station temperature. This is the pressure normally given in weather reports on radio, television, and newspapers or on the Internet. When barometers in the home are set to match the local weather reports, they measure pressure reduced to sea level, not the actual local atmospheric pressure. See Altimeter (barometer Vs. Absolute) . The reduction to sea level means that the ''normal range of fluctuations'' in pressure is the same for everyone. The pressures which are considered ''high pressure'' or ''low pressure'' do not depend on geographical location. This makes Isobar s on a weather map meaningful and useful tools. The altimeter setting in aviation, set either QNH or QFE , is another atmospheric pressure reduced to sea level, but the method of making this reduction differs slightly. See Altimeter .
Average ''sea-level pressure'' is 101.325 , where the Siberian High often attains a ''sea-level pressure'' above 1032.0 mbar. The lowest measurable ''sea-level pressure'' is found at the centers of hurricanes (typhoons, baguios) ALTITUDE ATMOSPHERIC PRESSURE VARIATION Pressure varies smoothly from the earth's surface to the top of the Mesosphere . Although the pressure changes with the weather, NASA has averaged the conditions for all parts of the earth year-round. The following is a list of air pressures (as a fraction of one atmosphere) with the corresponding average altitudes. The table gives a rough idea of air pressure at various altitudes. CALCULATING VARIATION WITH ALTITUDE See Also: Barometric formula There are two different equations for computing pressure at various height regimes below 86 km (or 278,400 feet). Equation 1 is used when the value of standard temperature lapse rate is not equal to zero and equation 2 is used when standard temperature lapse rate equals zero. Equation 1:
Equation 2:
where : = Static pressure (pascals) : = Standard temperature ( Kelvin s) : = Standard temperature lapse rate (kelvins per meter) : = Height above sea level (meters)
: = Gravitational constant (9.80665 m/s&2) : = Molar mass of Earth's air (28.9644 g/mol) Or converted to English units:Mechtly, E. A., 1973: ''The International System of Units, Physical Constants and Conversion Factors''. NASA SP-7012, Second Revision, National Aeronautics and Space Administration, Washington, D.C. where : = Static pressure (inches of mercury) : = Standard temperature ( Kelvin s) : = Standard temperature lapse rate (kelvins per foot) : = Height above sea level (feet)
: = Gravitational constant (32.17405 ft/s&2) : = Molar mass of Earth's air (28.9644 g/mol)
Sample Calculation: Find the pressure at 30,000 meters. First note that 30,000 meters is above 20,000 but below 32,000 so it therefore falls in the range of subscript b=2 in the chart above. Also note that the temperature lapse rate for that region is not equal to zero; therefore, equation 1 is appropriate.
Or : : : : Pascals at 30,000 meters LOCAL ATMOSPHERIC PRESSURE VARIATION on 19 October 2005 – 88.2 kPa in eye]] Atmospheric pressure varies widely on Earth , and these changes are important in studying Weather and Climate . See Pressure System for the effects of air pressure variations on weather. The highest recorded atmospheric pressure, 108.6 kPa (1086 mbar or 32.06 inches of mercury), occurred at Tosontsengel , Khövsgöl Province , Mongolia , 19 December , 2001 . 2 The lowest recorded non- Tornadic atmospheric pressure, 87.0 kPa (870 mbar or 25.69 inches of mercury), occurred in the Western Pacific during Typhoon Tip on 12 October , 1979 . 2 The record for the Atlantic ocean was 88.2 kPa (882 mbar or 26.04 inches of mercury) during Hurricane Wilma on 19 October 2005 . Atmospheric pressure shows a diurnal (twice-daily) cycle caused by global atmospheric tides. This effect is strongest in tropical zones, with amplitude of a few millibars, and almost zero in polar areas. A graph on the top of this page shows these rhythmic variations in northern Europe. These variations have two superimposed cycles, a circadian (24 h) cycle and semi-circadian (12 h) cycle. ATMOSPHERIC PRESSURE BASED ON HEIGHT OF WATER Atmospheric pressure is often measured with a mercury Barometer , and a height of approximately 760 mm (30 Inch es) of mercury is often used to teach, make visible, and illustrate (and measure) atmospheric pressure. However, since mercury is not a substance that humans commonly come in contact with, water often provides a more intuitive way to conceptualize the amount of pressure in one atmosphere. One atmosphere (101.325 kPa or 14.7 lbf/in&2) is the amount of pressure that can lift water approximately 10.3 m (33.9 feet). Thus, a diver at a depth 10.3 meters under water in a fresh-water lake experiences a pressure of about 2 atmospheres (1 atm for the air and 1 atm for the water). This is also the maximum height to which a column of water can be drawn up by Suction . Non-professional Barometer s are generally aneroid barometer (Figure 3) or Strain Gauge based. See Pressure Measurement for a description of barometers. SEE ALSO
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