Tuesday, 13 March 2012

Atmospheric pressure

Atmospheric burden is the force per assemblage breadth exerted into a apparent by the weight of air aloft that apparent in the atmosphere of Earth (or that of addition planet). In best affairs atmospheric burden is carefully approximated by the hydrostatic burden acquired by the accumulation of air aloft the altitude point. Low-pressure areas accept beneath atmospheric accumulation aloft their location, admitting high-pressure areas accept added atmospheric accumulation aloft their location. Likewise, as acclivity increases, there is beneath above atmospheric mass, so that burden decreases with accretion elevation. On average, a cavalcade of air one aboveboard centimeter in cross-section, abstinent from sea akin to the top of the atmosphere, has a accumulation of about 1.03 kg and weight of about 10.1 N (2.28 lbf) (A cavalcade one aboveboard inch in array would accept a accumulation of about 14.7 lbs and weight of about 65.4 N).

Standard atmospheric pressure

The accepted atmosphere (symbol: atm) is a assemblage of burden and is authentic as actuality according to 101.325 kPa.1 The afterward units are equivalent, but alone to the cardinal of decimal places displayed: 760 mmHg (torr), 29.92 inHg, 14.696 psi, 1013.25 millibars/hectopascal. One accepted is accepted burden acclimated for aeriform aqueous ability (ISO R554), and in the aerospace (ISO 2533) and petroleum (ISO 5024) industries. In 1971, the International Union of Pure and Applied Chemistry (IUPAC) said that for the purposes of allegorical the backdrop of substances, "the accepted pressure" should be authentic as absolutely 100 kPa (≈750.01 torr) or 29.53 inHg rather than the 101.325 kPa amount of “one accepted atmosphere”.2 This amount is acclimated as the accepted burden for the compressor and the aeriform apparatus industries (ISO 2787).3 (See additionally Accepted temperature and pressure.) In the United States, aeroembolism air breeze is generally abstinent in "standard cubic feet" per assemblage of time, area the "standard" agency the agnate abundance of air at accepted temperature and pressure. For every 1,000 anxiety you ascend, the atmospheric burden decreases by about 4%. However, this accepted atmosphere is authentic hardly differently: temperature = 20 °C (68 °F), air body = 1.225 kg/m³ (0.0765 lb/cu ft), distance = sea level, and about clamminess = 20%. In the air conditioner industry, the accepted is generally temperature = 0 °C (32 °F) instead. For accustomed gas, the Gas Processors Association (GPA) specifies a accepted temperature of 60 °F (15.6 °C), but allows a array of "base" pressures, including 14.65 psi (101.0 kPa), 14.656 psi (101.05 kPa), 14.73 psi (101.6 kPa) and 15.025 psi (103.59 kPa).4 For a accustomed "base" pressure, the college the air pressure, the colder it is; the lower the air pressure, the warmer it is.

Local atmospheric pressure variation

Atmospheric burden varies broadly on Earth, and these changes are important in belief acclimate and climate. See burden arrangement for the furnishings of air burden variations on weather.

Atmospheric burden shows a circadian or semidiurnal (twice-daily) aeon acquired by all-around atmospheric tides. This aftereffect is arch in close zones, with amplitude of a few millibars, and about aught in arctic areas. These variations accept two superimposed cycles, a circadian (24 h) aeon and semi-circadian (12 h) cycle.

Atmospheric pressure records

The accomplished barometric burden anytime recorded on Earth was 1,085.7 hectopascals (32.06 inHg) abstinent in Tonsontsengel, Mongolia on 19 December 2001.7 The everyman non-tornadic atmospheric burden anytime abstinent was 870 hPa (25.69 inches), set on 12 October 1979, during Typhoon Tip in the western Pacific Ocean. The altitude was based on an active ascertainment fabricated from a assay aircraft.8

Atmospheric pressure based on height of water

Atmospheric burden is generally abstinent with a mercury barometer, and a acme of about 760 millimetres (30 in) of mercury is generally acclimated to allegorize (and measure) atmospheric pressure. However, back mercury is not a actuality that bodies frequently appear in acquaintance with, baptize generally provides a added automatic way to anticipate the burden of one atmosphere.

One atmosphere (101 kPa or 14.7 psi) is the bulk of burden that can lift baptize about 10.3 m (34 ft). Thus, a diver 10.3 m underwater adventures a burden of about 2 atmospheres (1 atm of air additional 1 atm of water). This is additionally the best acme to which a cavalcade of baptize can be fatigued up by suction.

Low pressures such as accustomed gas curve are sometimes defined in inches of water, about accounting as w.c. (water column) or W.G. (inches baptize gauge). A archetypal gas application residential apparatus is rated for a best of 14 w.c., which is about 35 hPa.

In general, non-professional barometers are aneroid barometers or ache barometer based. See burden altitude for a description of barometers.

Boiling point of water

Water boils at about 100 °C (212 °F) at accepted atmospheric pressure. The baking point is the temperature at which the breath burden is according to the atmospheric burden about the water.9 Because of this, the baking point of baptize is lower at lower burden and college at college pressure. This is why affable at elevations added than 3,500 ft (1,100 m) aloft sea akin requires adjustments to recipes.10 A asperous approximation of acclivity can be acquired by barometer the temperature at which baptize boils; in the mid-19th century, this adjustment was acclimated by explorers.11