### Barometric pressure

"Air pressure" redirects here. For the pressure of air in other systems, see pressure.

Atmospheric pressure is the force per unit area exerted on a surface by the weight of air above that surface in the atmosphere of Earth (or that of another planet). In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. On a given plane, low-pressure areas have less atmospheric mass above their location, whereas high-pressure areas have more atmospheric mass above their location. Likewise, as elevation increases, there is less overlying atmospheric mass, so that atmospheric pressure decreases with increasing elevation. On average, a column of air one square centimeter in cross-section, measured from sea level to the top of the atmosphere, has a mass of about 1.03 kg and weight of about 10.1 N (2.28 lbf) (A column one square inch in cross-section would have a weight of about 14.7 lbs, or about 65.4 N).

## Standard atmospheric pressure

The standard atmosphere (symbol: atm) is a unit of pressure equal to 101325 Pa[1] or 1013.25 millibars or hectopascals. It is equivalent to 760 mmHg (torr), 29.92 inHg, 14.696 psi.

### Units of pressure related to or based on standard atmospheric pressure

• One standard is standard pressure used for pneumatic fluid power (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 specifying the properties of substances, "the standard pressure" should be defined as precisely 100 kPa (≈750.01 torr) or 29.53 inHg rather than the 101.325 kPa value of "one standard atmosphere".[2] This value is used as the standard pressure for the compressor and the pneumatic tool industries (ISO 2787).[3] (See also Standard temperature and pressure.)
• 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.
• For every 300 meters (≈1,000 feet) one ascends, the atmospheric pressure decreases by about 4%. However, this standard atmosphere is defined slightly differently: temperature = 20 °C (68 °F), air density = 1.225 kg/m³ (0.0765 lb/cu ft), altitude = sea level, and relative humidity = 20%.
• In the air conditioner industry, the standard is often temperature = 0 °C (32 °F) instead. For natural gas, the Gas Processors Association (GPA) specifies a standard temperature of 60 °F (15.6 °C), but allows a variety 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 given "base" pressure, the higher the air pressure, the colder it is; the lower the air pressure, the warmer it is.

## Mean sea level pressure

The mean sea level pressure (MSLP) is the atmospheric pressure at sea level or (when measured at a given elevation on land) the station pressure reduced to sea level assuming that the temperature falls at a lapse rate of 6.5 K per km in the fictive layer of air between the station and sea level.

This is the atmospheric 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 atmospheric pressure is the same for everyone. The pressures that 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.

QNH
The barometric altimeter setting that will cause the altimeter to read airfield elevation when on the airfield. In ISA temperature conditions the altimeter will read altitude above mean sea level in the vicinity of the airfield
QFE
The barometric altimeter setting that will cause an altimeter to read zero when at the reference datum of a particular airfield (in general, a runway threshold). In ISA temperature conditions the altimeter will read height above the datum in the vicinity of the airfield.

QFE and QNH are arbitrary Q codes rather than abbreviations, but the mnemonics "Nautical Height" (for QNH) and "Field Elevation" (for QFE) are often used by pilots to distinguish them.

Average sea-level pressure is 101.325 kPa (1013.25 mbar, or hPa) or 29.92 inches (inHg) or 760 millimetres of mercury (mmHg). In aviation weather reports (METAR), QNH is transmitted around the world in millibars or hectopascals (1 millibar = 1 hectopascal), except in the United States, Canada, and Colombia where it is reported in inches (to two decimal places) of mercury. (The United States and Canada also report 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.[5] However, in Canada's public weather reports, sea level pressure is instead reported in kilopascals,[6] while Environment Canada's standard unit of pressure is the same.[7][8])

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 1050.0 mbar (105.00 kPa, 30.01 inHg), with record highs close to 1085.0 mbar (108.50 kPa, 32.04 inHg). The lowest measurable sea-level pressure is found at the centers of tropical cyclones and tornadoes, with a record low of 870 mbar (87 kPa) (see Atmospheric pressure records).

## 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. As altitude increases, atmospheric pressure decreases. One can calculate the atmospheric pressure at a given altitude.[9] Temperature and humidity also affect the atmospheric pressure, and it is necessary to know these to compute an accurate figure. The graph at right was developed for a temperature of 15 °C and a relative humidity of 0%.

At low altitudes above the sea level, the pressure decreases by cca 1.2 kPa for every 100 meters. For higher altitudes within the troposphere, the following equation (the Barometric formula) relates atmospheric pressure p to altitude h

$p = p_0 \cdot \left\left(1 - \frac\left\{L \cdot h\right\}\left\{T_0\right\} \right\right)^\frac\left\{g \cdot M\right\}\left\{R \cdot L\right\} \approx p_0 \cdot \exp \left\left(- \frac\left\{g \cdot M \cdot h\right\}\left\{R \cdot T_0\right\} \right\right),$

where the constant parameters are as described below:

Parameter Description Value
p0 sea level standard atmospheric pressure 101325 Pa
L temperature lapse rate 0.0065 K/m
T0 sea level standard temperature 288.15 K
g Earth-surface gravitational acceleration 9.80665 m/s2
M molar mass of dry air 0.0289644 kg/mol
R universal gas constant 8.31447 J/(mol•K)

## Local atmospheric pressure variation

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.

Atmospheric pressure shows a diurnal or semidiurnal (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. These variations have two superimposed cycles, a circadian (24 h) cycle and semi-circadian (12 h) cycle.

## Atmospheric pressure records

The highest adjusted-to-sea level barometric pressure ever recorded on Earth (above 750 meters) was 1,085.7 hectopascals (32.06 inHg) measured in Tosontsengel, Mongolia on 19 December 2001.[10] The highest adjusted-to-sea level barometric pressure ever recorded (below 750 meters) was at Agata, Evenhiyskiy, Russia [66°53’N, 93°28’E, elevation: 261 m (856.3 ft)] on 31 December 1968 of 1,083.3 hectopascals (31.99 inHg).[11] The discrimination is due to the problematic assumptions (assuming a standard lapse rate) associated with reduction of sea level from high elevations.[10] The lowest non-tornadic atmospheric pressure ever measured was 870 hPa (25.69 inHg), set on 12 October 1979, during Typhoon Tip in the western Pacific Ocean. The measurement was based on an instrumental observation made from a reconnaissance aircraft.[12] The normal high barometric pressure at the Dead Sea, as measured by a standard mercury manometer and blood gas analyzer, was found to be 799 mmHg (1065 hPa).[13]

## Atmospheric pressure based on depth of water

Atmospheric pressure is often measured with a mercury barometer, and a height of approximately 760 millimetres (30 in) of mercury is often used to illustrate (and measure) atmospheric pressure. However, since mercury is not a familiar substance to most people, water may provide a more intuitive way to visualize the pressure of one atmosphere.

One atmosphere (101 kPa or 14.7 psi) is the pressure caused by the weight of a column of fresh water of approximately 10.3 m (34 ft). Thus, a diver 10.3 m underwater experiences a pressure of about 2 atmospheres (1 atm of air plus 1 atm of water). This is the maximum height to which a column of water can be drawn up by suction at atmospheric pressure.

Low pressures such as natural gas lines are sometimes specified in inches of water, typically written as w.c. (water column) or W.G. (inches water gauge). A typical gas-using residential appliance is rated for a maximum of 14 w.c., which is approximately 35 hPa.

In general, non-professional barometers are aneroid barometers or strain gauge based. See pressure measurement for a description of barometers.

## Boiling point of water

Clean fresh water boils at about 100 °C (212 °F) at standard atmospheric pressure. The boiling point is the temperature at which the vapor pressure is equal to the atmospheric pressure around the water.[14] Because of this, the boiling point of water is lower at lower pressure and higher at higher pressure. This is why cooking at elevations more than 1,100 m (3,600 ft) above sea level requires adjustments to recipes.[15] A rough approximation of elevation can be obtained by measuring the temperature at which water boils; in the mid-19th century, this method was used by explorers.[16]