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Atmospheric pressure

Atmospheric pressure is the pressure above any area in the Earth’s atmosphere caused by the weight of air. Standard atmospheric pressure (atm) is discussed in the next section.

Air masses are affected by the general atmospheric pressure within the mass, creating areas of high pressure (anti-cyclones) and low pressure (depressions).

As elevation increases, fewer air molecules are above. Therefore, atmospheric pressure decreases with increasing altitude. The following relationship is a first-order approximation:

Atmospheric pressure usa

 

where P is the pressure in pascals and h the height in metres. This shows that the pressure at an altitude of 31 km is about 10(5-2) Pa = 1000 Pa, or 1% of that at sea level1.

A column of air, 1 square inch in cross section, measured from sea level to the top of the atmosphere would weigh approximately 14.7 lb. A 1 m2 column of air would weigh about 100 kilonewtons. See density of air.

 

Standard atmospheric pressure

Standard atmospheric pressure or “the standard atmosphere” (1 atm) is defined as 101.325 kilopascals (kPa). (see also Standard temperature and pressure)

This can also be stated as:

29 117/127 inches of mercury ≈ 29.92 inHg
760 millimetres of mercury (mmHg) or torrs (Torr)
1013.25 millibars (mbar, also mb) or hectopascals (hPa)
14.6959 psia or 0 psig (pounds-force per square inch, absolute or gauge) (lbf/in²)
2116.2 pounds-force per square foot (lbf/ft²)
1 6517/196133 technical atmospheres (at) ≈ 1.03322745 at
This “standard pressure” is a purely arbitrary representative value for pressure at sea level, and real atmospheric pressures vary from place to place and moment to moment everywhere in the world.

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 = 0.075 lb/ft³ (1.20 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).

Mean sea level pressure (MSLP or SLP)
Mean sea level pressure (MSLP or SLP) is the pressure at sea level or (when measured at a given height on land) the station pressure reduced to sea level by an appropriate altitude dependant formula.

This is the pressure normally given in weather reports on radio, television, and newspapers. 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.

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 isobars 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.

QNH barometric altimeter setting which will cause the altimeter to read altitude above mean sea level in the vicinity of an airfield.
QFE barometric altimeter setting which will cause an altimeter to read height above the official reference datum of a particular airfield (generally a runway threshold).
Average sea-level pressure is 1013.25 hPa (mbar) or 29.921 inches of mercury (inHg). In aviation weather reports (METAR), QNH is transmitted around the world in millibars or hectopascals, except in the United States and Canada where it is reported in inches (or hundredths of inches) of mercury. (The United States also reports sea level pressure SLP, which is reduced to sea level by a different method, in the remarks section, not an internationally transmitted part of the code, in hectopascals or millibars. In Canada’s public weather reports, sea level pressure is reported in kilopascals, while Environment Canada’s standard unit of pressure is hectopascal.) In the weather code, three digits are all that is needed, Decimal points and the one or two most significant digits are omitted: 1013.2 mbar or 101.32 kPa is transmitted as 132; 1000.0 mbar or 100.00 kPa is transmitted as 000; 998.7 mbar or 99.87 kPa is transmitted as 987; etc. The highest sea-level pressure on Earth occurs in Siberia, 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).

Atmospheric pressure variation

Hurricane Wilma on 19 October 2005 – 88.2 kPa in eye
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Hurricane Wilma on 19 October 2005 – 88.2 kPa in eye
Atmospheric pressure varies widely on the Earth, and these variations 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, Mongolia, 19 December 20012.

The lowest recorded non-tornadic atmospheric pressure, 87.0 kPa (870 mbar or 25.69 inHg), occurred in the Western Pacific during Typhoon Tip on 12 October 19792. The record for the Atlantic ocean was 88.2 kPa (882 mbar or 26.04 inHg) during Hurricane Wilma on 19 October 2005.

Atmospheric pressure shows a diurnal (daily) rhythm. This effect is very strong in tropical zones, and almost zero in polar areas. A graph shows these rhymic variations in northern Europe on the top of this page. In tropical zones it may reach above 5 mbar variation. These variations follow a circadian (24 h) and at the same time semi-circadian (12 h) rhythm.

Intuitive feeling for atmospheric pressure based on height of water
Atmospheric pressure is often measured with a mercury barometer, and a height of approximately 760 mm (30 inches) 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/in²) is the amount of pressure that can lift water approximately 10.3 m (33.9 feet). Thus, a diver at a depth 10.3 metres 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).

In terms of city water pressure, one atmosphere is approximately one-half to one-fifth the pressure of typical city water mains (i.e., water pressure is around 2 to 5 atmospheres).

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