Atmospheric pressure, also known as barometric pressure (after the
barometer), is the
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
within the
atmosphere of Earth. The
standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25
millibars, 760
mm Hg
A millimetre of mercury is a manometric unit of pressure, formerly defined as the extra pressure generated by a column of mercury one millimetre high, and currently defined as exactly pascals. It is denoted mmHg or mm Hg.
Although not an S ...
, 29.9212
inchesHg, or 14.696
psi
Psi, PSI or Ψ may refer to:
Alphabetic letters
* Psi (Greek) (Ψ, ψ), the 23rd letter of the Greek alphabet
* Psi (Cyrillic) (Ѱ, ѱ), letter of the early Cyrillic alphabet, adopted from Greek
Arts and entertainment
* "Psi" as an abbreviation ...
.
[International Civil Aviation Organization. ''Manual of the ICAO Standard Atmosphere'', Doc 7488-CD, Third Edition, 1993. .] The atm unit is roughly equivalent to the
mean sea-level
Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardised g ...
atmospheric pressure on Earth; that is, the Earth's atmospheric pressure at sea level is approximately 1 atm.
In most circumstances, atmospheric pressure is closely approximated by the
hydrostatic pressure
Fluid statics or hydrostatics is the branch of fluid mechanics that studies the condition of the equilibrium of a floating body and submerged body "fluids at hydrostatic equilibrium and the pressure in a fluid, or exerted by a fluid, on an imme ...
caused by the
weight
In science and engineering, the weight of an object is the force acting on the object due to gravity.
Some standard textbooks define weight as a Euclidean vector, vector quantity, the gravitational force acting on the object. Others define weigh ...
of
air above the measurement point. As
elevation
The elevation of a geographic location is its height above or below a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational surface (see Geodetic datum § Vert ...
increases, there is less overlying atmospheric mass, so atmospheric pressure decreases with increasing elevation. Because the atmosphere is thin relative to the Earth's radius—especially the dense atmospheric layer at low altitudes—the Earth's
gravitational acceleration
In physics, gravitational acceleration is the acceleration of an object in free fall within a vacuum (and thus without experiencing drag). This is the steady gain in speed caused exclusively by the force of gravitational attraction. All bodies ...
as a function of altitude can be approximated as constant and contributes little to this fall-off. Pressure measures force per unit area, with
SI units
The International System of Units, known by the international abbreviation SI in all languages and sometimes Pleonasm#Acronyms and initialisms, pleonastically as the SI system, is the modern form of the metric system and the world's most wid ...
of
pascals
The pascal (symbol: Pa) is the unit of pressure in the International System of Units (SI), and is also used to quantify internal pressure, stress, Young's modulus, and ultimate tensile strength. The unit, named after Blaise Pascal, is defined as ...
(1 pascal = 1
newton
Newton most commonly refers to:
* Isaac Newton (1642–1726/1727), English scientist
* Newton (unit), SI unit of force named after Isaac Newton
Newton may also refer to:
Arts and entertainment
* ''Newton'' (film), a 2017 Indian film
* Newton ( ...
per square
metre
The metre (British spelling) or meter (American spelling; see spelling differences) (from the French unit , from the Greek noun , "measure"), symbol m, is the primary unit of length in the International System of Units (SI), though its pref ...
, 1N/m
2). On average, a column of air with a cross-sectional area of 1 square
centimetre (cm
2), measured from the mean (average) sea level to the top of Earth's atmosphere, has a
mass
Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementar ...
of about 1.03
kilogram
The kilogram (also kilogramme) is the unit of mass in the International System of Units (SI), having the unit symbol kg. It is a widely used measure in science, engineering and commerce worldwide, and is often simply called a kilo colloquially ...
and exerts a
force
In physics, a force is an influence that can change the motion of an object. A force can cause an object with mass to change its velocity (e.g. moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a p ...
or "weight" of about 10.1
newtons
The newton (symbol: N) is the unit of force in the International System of Units (SI). It is defined as 1 kg⋅m/s, the force which gives a mass of 1 kilogram an acceleration of 1 metre per second per second. It is named after Isaac Newton in r ...
, resulting in a pressure of 10.1 N/cm
2 or 101
kN/m
2 (101 kilopascals, kPa). A column of air with a cross-sectional area of 1in
2 would have a weight of about 14.7lb
f, resulting in a pressure of 14.7lb
f/in
2.
Mechanism
Atmospheric pressure is caused by the gravitational attraction of the planet on the atmospheric gases above the surface and is a function of the mass of the planet, the radius of the surface, and the amount and composition of the gases and their vertical distribution in the atmosphere. It is modified by the planetary rotation and local effects such as wind velocity, density variations due to temperature and variations in composition.
Mean sea-level pressure
![Mslp-jja-djf](https://upload.wikimedia.org/wikipedia/commons/1/17/Mslp-jja-djf.png)
The ''mean sea-level pressure'' (MSLP) is the atmospheric pressure at
mean sea level
There are several kinds of mean in mathematics
Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. ...
(PMSL). This is the atmospheric pressure normally given in weather reports on radio, television, and newspapers or on the
Internet
The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a '' network of networks'' that consists of private, pub ...
. When
barometers in the home are set to match the local weather reports, they display pressure adjusted to sea level, not the actual local atmospheric pressure.
The ''
altimeter setting'' in aviation is an atmospheric pressure adjustment.
Average ''sea-level pressure'' is . In aviation weather reports (
METAR
METAR is a format for reporting weather information. A METAR weather report is predominantly used by aircraft pilots, and by meteorologists, who use aggregated METAR information to assist in weather forecasting.
Raw METAR is the most common form ...
),
QNH
The Q-code is a standardised collection of three-letter codes that each start with the letter "Q". It is an operating signal initially developed for commercial radiotelegraph communication and later adopted by other radio services, especially ...
is transmitted around the world in hectopascals or millibars (1 hectopascal = 1 millibar), except in the
United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...
,
Canada
Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tot ...
, and
Japan
Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...
where it is reported in
inches of mercury (to two decimal places). The United States and Canada also report ''sea-level pressure'' SLP, which is adjusted to sea level by a different method, in the remarks section, not in the internationally transmitted part of the code, in hectopascals or millibars. However, in Canada's public weather reports, sea level pressure is instead reported in kilopascals.
In the US weather code remarks, three digits are all that are transmitted; decimal points and the one or two most significant digits are omitted: is transmitted as 132; is transmitted as 000; 998.7hPa is transmitted as 987; etc. The highest ''sea-level pressure'' on
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
occurs in
Siberia
Siberia ( ; rus, Сибирь, r=Sibir', p=sʲɪˈbʲirʲ, a=Ru-Сибирь.ogg) is an extensive geographical region, constituting all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has been a part of ...
, where the
Siberian High often attains a ''sea-level pressure'' above , with record highs close to . The lowest measurable ''sea-level pressure'' is found at the centres of
tropical cyclone
A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depend ...
s and
tornado
A tornado is a violently rotating column of air that is in contact with both the surface of the Earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. It is often referred to as a twister, whirlwind or cyclone, altho ...
es, with a
record low of .
Surface pressure
''Surface pressure'' is the atmospheric pressure at a location on
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
's surface (
terrain
Terrain or relief (also topographical relief) involves the vertical and horizontal dimensions of land surface. The term bathymetry is used to describe underwater relief, while hypsometry studies terrain relative to sea level. The Latin word ...
and
oceans
The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the worl ...
). It is directly proportional to the
mass
Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementar ...
of air over that location.
For numerical reasons, atmospheric models such as
general circulation model
A general circulation model (GCM) is a type of climate model. It employs a mathematical model of the general circulation of a planetary atmosphere or ocean. It uses the Navier–Stokes equations on a rotating sphere with thermodynamic terms f ...
s (GCMs) usually predict the nondimensional ''
logarithm
In mathematics, the logarithm is the inverse function to exponentiation. That means the logarithm of a number to the base is the exponent to which must be raised, to produce . For example, since , the ''logarithm base'' 10 o ...
of surface pressure''.
The average value of surface pressure on Earth is 985 hPa.
[Jacob, Daniel J]
Introduction to Atmospheric Chemistry
. Princeton University Press, 1999. This is in contrast to mean sea-level pressure, which involves the extrapolation of pressure to sea level for locations above or below sea level. The average pressure at mean sea level (
MSL) in the International Standard Atmosphere (
ISA
Isa or ISA may refer to:
Places
* Isa, Amur Oblast, Russia
* Isa, Kagoshima, Japan
* Isa, Nigeria
* Isa District, Kagoshima, former district in Japan
* Isa Town, middle class town located in Bahrain
* Mount Isa, Queensland, Australia
* Mount Is ...
) is 1013.25 hPa, or 1 atmosphere (atm), or 29.92 inches of mercury.
Pressure (P), mass (m), and acceleration due to gravity (g) are related by P = F/A = (m*g)/A, where A is the surface area. Atmospheric pressure is thus proportional to the weight per unit area of the atmospheric mass above that location.
Altitude variation
![Atmospheric Pressure vs](https://upload.wikimedia.org/wikipedia/commons/8/88/Atmospheric_Pressure_vs._Altitude.png)
Pressure on Earth varies with the altitude of the surface, so air pressure on mountains is usually lower than air pressure at sea level. Pressure varies smoothly from the Earth's surface to the top of the
mesosphere
The mesosphere (; ) is the third layer of the atmosphere, directly above the stratosphere and directly below the thermosphere. In the mesosphere, temperature decreases as altitude increases. This characteristic is used to define its limits: it ...
. 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. Temperature and humidity also affect the atmospheric pressure. Pressure is proportional to temperature and inversely proportional to humidity. And it is necessary to know both of these to compute an accurate figure. The graph was developed for a temperature of 15 °C and a relative humidity of 0%.
At low altitudes above sea level, the pressure decreases by about for every 100 metres. For higher altitudes within the
troposphere
The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...
, the following equation (the
barometric formula) relates atmospheric pressure ''p'' to altitude ''h'':
. The values in these equations are:
Local variation
![Wilma1315z-051019-1kg12](https://upload.wikimedia.org/wikipedia/commons/f/fa/Wilma1315z-051019-1kg12.jpg)
Atmospheric pressure varies widely on Earth, and these changes are important in studying
weather
Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmosphere, the ...
and
climate
Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologic ...
. 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
Atmospheric tides are global-scale periodic oscillations of the atmosphere. In many ways they are analogous to ocean tides. Atmospheric tides can be excited by:
*The regular day–night cycle in the Sun's heating of the atmosphere ( insolation) ...
. This effect is strongest in tropical zones, with an amplitude of a few hectopascals, and almost zero in polar areas. These variations have two superimposed cycles, a circadian (24 h) cycle and a semi-circadian (12 h) cycle.
Records
The highest adjusted-to-sea level barometric pressure ever recorded on Earth (above 750 meters) was measured in
Tosontsengel, Mongolia on 19 December 2001.
The highest adjusted-to-sea level barometric pressure ever recorded (below 750 meters) was at Agata in
Evenk Autonomous Okrug
Evenk Autonomous Okrug (russian: Эвенки́йский автоно́мный о́круг, ; , ), or Evenkia, was a federal subject of Russia (an autonomous okrug of Krasnoyarsk Krai). It had been created in 1930. Its administrative center w ...
, Russia (66°53'N, 93°28'E, elevation: ) on 31 December 1968 of . The discrimination is due to the problematic assumptions (assuming a standard lapse rate) associated with reduction of sea level from high elevations.
The
Dead Sea
The Dead Sea ( he, יַם הַמֶּלַח, ''Yam hamMelaḥ''; ar, اَلْبَحْرُ الْمَيْتُ, ''Āl-Baḥrū l-Maytū''), also known by other names, is a salt lake bordered by Jordan to the east and Israel and the West Bank ...
, the lowest place on Earth at below sea level, has a correspondingly high typical atmospheric pressure of 1065hPa. A below-sea-level surface pressure record of was set on 21 February 1961.
The lowest non-tornadic atmospheric pressure ever measured was 870 hPa (0.858 atm; 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.
Measurement based on the depth of water
One
atmosphere
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
() is also the pressure caused by the weight of a column of freshwater of approximately . Thus, a diver 10.3 m underwater experiences a pressure of about 2 atmospheres (1 atm of air plus 1 atm of water). Conversely, 10.3 m is the maximum height to which water can be raised using
suction
Suction is the colloquial term to describe the air pressure differential between areas.
Removing air from a space results in a pressure differential. Suction pressure is therefore limited by external air pressure. Even a perfect vacuum cannot ...
under standard atmospheric conditions.
Low pressures, such as
natural gas
Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbo ...
lines, are sometimes specified in
inches of water, typically written as ''w.c.'' (water column) gauge or ''w.g.'' (inches water) gauge. A typical gas-using residential appliance in the US is rated for a maximum of , which is approximately 14 w.g. Similar metric units with a wide variety of names and notation based on
millimetres,
centimetres or metres are now less commonly used.
Boiling point of liquids
![Kochendes wasser02](https://upload.wikimedia.org/wikipedia/commons/1/18/Kochendes_wasser02.jpg)
Pure water
boils
A boil, also called a furuncle, is a deep folliculitis, which is an infection of the hair follicle. It is most commonly caused by infection by the bacterium ''Staphylococcus aureus'', resulting in a painful swollen area on the skin caused by an ...
at at earth's standard atmospheric pressure. The boiling point is the temperature at which the
vapour pressure is equal to the atmospheric pressure around the liquid. Because of this, the boiling point of liquids is lower at lower pressure and higher at higher pressure. Cooking at high elevations, therefore, requires adjustments to recipes or
pressure cooking
Pressure cooking is the process of cooking food under high pressure steam and water or a water-based cooking liquid, in a sealed vessel known as a ''pressure cooker''. High pressure limits boiling, and creates higher cooking temperatures which c ...
. 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. Conversely, if one wishes to evaporate a liquid at a lower temperature, for example in
distillation
Distillation, or classical distillation, is the process of separation process, separating the components or substances from a liquid mixture by using selective boiling and condensation, usually inside an apparatus known as a still. Dry distilla ...
, the atmospheric pressure may be lowered by using a
vacuum pump
A vacuum pump is a device that draws gas molecules from a sealed volume in order to leave behind a partial vacuum. The job of a vacuum pump is to generate a relative vacuum within a capacity. The first vacuum pump was invented in 1650 by Otto v ...
, as in a
rotary evaporator
A rotary evaporator (rotovap) is a device used in chemical laboratories for the efficient and gentle removal of solvents from samples by evaporation. When referenced in the chemistry research literature, description of the use of this technique and ...
.
Measurement and maps
An important application of the knowledge that atmospheric pressure varies directly with altitude was in determining the height of hills and mountains, thanks to reliable pressure measurement devices. In 1774,
Maskelyne was
confirming Newton's theory of gravitation at and on
Schiehallion mountain in Scotland, and he needed to measure elevations on the mountain's sides accurately.
William Roy
Major-General William Roy (4 May 17261 July 1790) was a Scottish military engineer, surveyor, and antiquarian. He was an innovator who applied new scientific discoveries and newly emerging technologies to the accurate geodetic mapping of Gr ...
, using barometric pressure, was able to confirm Maskelyne's height determinations, the agreement being to be within one meter (3.28 feet). This method became and continues to be useful for survey work and map making.
[Hewitt, Rachel, ''Map of a Nation – a Biography of the Ordnance Survey'' ]
See also
*
*
*
* – physical damage to body tissues caused by a difference in pressure between an air space inside or beside the body and the surrounding gas or liquid.
*
*
*
Collapsing can
Collapsing can or can crusher experiment is a demonstration of an aluminum can being crushed by atmospheric pressure. Due to the low pressure inside a can as compared to the pressure outside, the pressure outside exerts a force on the can causing ...
– an aluminium can is crushed by the atmospheric pressure surrounding it
*
*
* , a tabulation of typical variations of principal thermodynamic variables of the atmosphere (pressure, density, temperature, etc.) with altitude, at middle latitudes.
*
*
* , an empirical, global reference atmospheric model of the Earth from ground to space
*
*
*
*
*
References
External links
1976 Standard Atmospherefrom
NASA
The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research.
NASA was established in 1958, succeeding t ...
Source code and equations for the 1976 Standard AtmosphereCalculator using multiple units and properties for the 1976 Standard Atmosphere*
ttps://earth.nullschool.net/#current/wind/surface/level/overlay=mean_sea_level_pressure/winkel3 Current map of global mean sea-level pressurebr>
Calculate pressure from altitude and vice versa
Experiments
Georgia State University's
HyperPhysics website – requires QuickTimeTest showing a can being crushed after boiling water inside it, then moving it into a tub of ice-cold water.
{{authority control
Atmosphere
Atmospheric thermodynamics
Pressure
Underwater diving physics