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Atmospheric pressure
Atmospheric pressure
Atmospheric pressure
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Atmospheric pressure, also known as air pressure or barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as 101,325 Pa (1,013.25 hPa), which is equivalent to 1,013.25 millibars, 760 mm Hg, 29.9212 inches Hg, or 14.696 psi.[1] The atm unit is roughly equivalent to the mean sea-level 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 caused by the weight of air above the measurement point. As elevation 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 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 of pascals (1 pascal = 1 newton per square metre, 1 N/m2). On average, a column of air with a cross-sectional area of 1 square centimetre (cm2), measured from the mean (average) sea level to the top of Earth's atmosphere, has a mass of about 1.03 kilogram and exerts a force or "weight" of about 10.1 newtons, resulting in a pressure of 10.1 N/cm2 or 101 kN/m2 (101 kilopascals, kPa). A column of air with a cross-sectional area of 1 in2 would have a weight of about 14.7 lbf, resulting in a pressure of 14.7 lbf/in2.

Mechanism

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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.[2][3] It is modified by the planetary rotation and local effects such as wind velocity, density variations due to temperature and variations in composition.[4]

Mean sea-level pressure

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Map showing atmospheric pressure in mbar or hPa
15-year average mean sea-level pressure for June, July, and August (top) and December, January, and February (bottom). ERA-15 re-analysis.
Kollsman-type barometric aircraft altimeter.

The mean sea-level pressure (MSLP) is the atmospheric pressure at mean sea level. This is the atmospheric pressure normally given in weather reports via meteorologists on radio, television, and newspapers or on the Internet.[5]

The altimeter setting in aviation is an atmospheric pressure adjustment.

Average sea-level pressure is 1,013.25 hPa (29.921 inHg; 760.00 mmHg). In aviation weather reports (METAR), QNH is transmitted around the world in hectopascals or millibars (1 hectopascal = 1 millibar). In the United States, Canada, and Japan altimeter setting 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.[6] However, in Canada's public weather reports, sea level pressure is instead reported in kilopascals.[7] 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: 1,013.2 hPa (14.695 psi) is transmitted as 132; 1,000 hPa (100 kPa) is transmitted as 000; 998.7 hPa is transmitted as 987; etc. A system transmitting the last three digits transmits the same code (800) for 1080.0 hPa as for 980.0 hPa.

The highest sea-level pressure on Earth occurs in Siberia, where the Siberian High often attains a sea-level pressure above 1,050 hPa (15.2 psi; 31 inHg), with record highs close to 1,085 hPa (15.74 psi; 32.0 inHg). The lowest measurable sea-level pressure is found at the centres of tropical cyclones and tornadoes, with a record low of 870 hPa (12.6 psi; 26 inHg).

Surface pressure

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This section is about the atmospheric surface pressure. For other uses, see Surface pressure (disambiguation).

Surface pressure is the atmospheric pressure at a location on Earth's surface (terrain and oceans). It is directly proportional to the mass of air over that location.

For numerical reasons, atmospheric models such as general circulation models (GCMs) usually predict the nondimensional logarithm of surface pressure.

The average value of surface pressure on Earth is 985 hPa.[8] 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) is 1,013.25 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

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See also: Blaise Pascal § First atmospheric pressure vs. altitude experiment
Further information: Barometric formula and Vertical pressure variation
Cloud formation above Snæfellsjökull (Iceland), formed above the mountain by orographic lift
Variation in atmospheric pressure with altitude, computed for 15 °C and 0% relative humidity.
This plastic bottle was sealed at approximately 4,300 metres (14,000 ft) altitude, and was crushed by the increase in atmospheric pressure, recorded at 2,700 metres (9,000 ft) and 300 metres (1,000 ft), as it was brought down towards sea level.

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. 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. Pressure is proportional to temperature and inversely related to humidity, and both of these are necessary to compute an accurate figure. The graph on the rightabove 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 1.2 kPa (12 hPa) for every 100 metres. For higher altitudes within the troposphere, the following equation (the barometric formula) relates atmospheric pressure p to altitude h:


The values in these equations are:

Parameter Description Value
h Height above mean sea level  m
p0 Sea level standard atmospheric pressure 101,325 Pa
L Temperature lapse rate, = g/cp for dry air ~ 0.00976 K/m
cp Constant-pressure specific heat 1,004.68506 J/(kg·K)
T0 Sea level standard temperature 288.15 K
g Earth-surface gravitational acceleration 9.80665 m/s2
M Molar mass of dry air 0.02896968 kg/mol
R0 Universal gas constant 8.314462618 J/(mol·K)

Local variation

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Hurricane Wilma on 19 October 2005. The pressure in the eye of the storm was 882 hPa (12.79 psi) at the time the image was taken.

Atmospheric pressure varies widely on Earth, and these changes are important in studying weather and climate. Atmospheric pressure shows a diurnal or semidiurnal (twice-daily) cycle caused by global atmospheric tides. 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

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The highest adjusted-to-sea level barometric pressure ever recorded on Earth (above 750 meters) was 1,084.8 hPa (32.03 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 in Evenk Autonomous Okrug, Russia (66°53' N, 93°28' E, elevation: 261 m, 856 ft) on 31 December 1968 of 1,083.8 hPa (32.005 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 Dead Sea, the lowest place on Earth at 430 metres (1,410 ft) below sea level, has a correspondingly high typical atmospheric pressure of 1,065 hPa.[12] A below-sea-level surface pressure record of 1,081.8 hPa (31.95 inHg) was set on 21 February 1961.[13]

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.[14]

Measurement based on the depth of water

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One atmosphere (101.325 kPa or 14.7 psi) is also the pressure caused by the weight of a column of freshwater of approximately 10.3 m (33.8 ft). Thus, a diver 10.3 m under water 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 under standard atmospheric conditions.

Low pressures, such as natural gas 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 1⁄2 psi (3.4 kPa; 34 mbar), 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

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Boiling water

Pure water boils at 100 °C (212 °F) 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.[15] 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[16] or pressure cooking. 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.[17] Conversely, if one wishes to evaporate a liquid at a lower temperature, for example in distillation, the atmospheric pressure may be lowered by using a vacuum pump, as in a rotary evaporator.

Measurement and maps

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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, Nevil 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. This event is known as the Schiehallion experiment. William Roy, using barometric pressure, was able to confirm Maskelyne's height determinations; the agreement was within one meter (3.28 feet). This method became and continues to be useful for survey work and map making.[18]

See also

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  • Atmospheric density – Mass per unit volume of the Earth's atmosphere
  • Atmosphere of Earth – Gas layer surrounding Earth
  • Barometric formula – Formula used to model how air pressure varies with altitude
  • Barotrauma – Injury due to pressure difference between gas filled space and adjoining tissue – 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.
  • Cabin pressurization – Process to maintain internal air pressure in aircraft or spacecraft
  • Cavitation – Low-pressure voids formed in liquids
  • Collapsing can – an aluminium can is crushed by the atmospheric pressure surrounding it
  • Effects of high altitude on humans – Environmental effects on physiology and mental health
  • High-pressure area – Region with higher atmospheric pressure
  • International Standard Atmosphere – Atmospheric model, a tabulation of typical variations of principal thermodynamic variables of the atmosphere (pressure, density, temperature, etc.) with altitude, at middle latitudes.
  • Low-pressure area – Area with air pressures lower than adjacent areas
  • Meteorology – Interdisciplinary scientific study of the atmosphere focusing on weather forecasting
  • NRLMSISE-00, an empirical, global reference atmospheric model of the Earth from ground to space
  • Plenum chamber – Chamber containing a fluid under pressure
  • Pressure – Force distributed over an area
  • Pressure measurement
  • Standard atmosphere (unit) – Unit of pressure defined as 101325 Pa
  • Horse latitudes – Latitudes 30–35 degrees north and south of the Equator

References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Atmospheric pressure

View on Grokipedia
from Grokipedia
Atmospheric pressure is the force per unit area exerted by the weight of air molecules in Earth's atmosphere on a surface below, arising from the cumulative effect of , , and . At under standard conditions, it equals 1013.25 hectopascals (hPa), equivalent to 1013.25 millibars (mb), 29.92 inches of mercury (inHg), or 14.7 pounds per (psi). This pressure is measured using an instrument called a , which detects the force of air molecules colliding with its surface. Atmospheric pressure decreases with increasing altitude because the mass of air above diminishes, with roughly half of the atmosphere's molecules contained within the first 5.6 kilometers (18,000 feet) above the surface. It also varies horizontally due to factors such as temperature—warm air is less dense and exerts lower pressure, while cold air is denser and exerts higher pressure—and moisture content, as moist air is less dense than dry air. Daily fluctuations occur due to solar heating, with pressure typically lowest around 4 a.m. and 4 p.m. and highest around 10 a.m. and 10 p.m. For meteorological purposes, observed pressures are often reduced to sea-level equivalents to enable consistent comparisons across different elevations. Variations in atmospheric pressure play a central role in driving global patterns, as differences in pressure create horizontal forces that generate winds. High-pressure systems (anticyclones), marked by sinking air and higher central pressure, are associated with clear, fair and outward-spiraling winds that rotate clockwise in the . In contrast, low-pressure systems (cyclones), characterized by rising air and lower central pressure, promote cloud formation, , and stormy conditions, with inward-spiraling winds rotating counterclockwise in the . These pressure gradients, influenced by uneven solar heating of Earth's surface, are fundamental to the formation of fronts, storms, and circulation patterns like the and jet streams. Beyond weather, atmospheric pressure affects numerous practical domains, including —where altimeters rely on pressure changes to estimate altitude—and human physiology, as rapid decreases at high altitudes can lead to conditions like hypoxia. In engineering and , understanding pressure variations is essential for designing structures, predicting ocean influenced by atmospheric loading, and modeling dynamics.

Fundamentals

Definition

Atmospheric pressure is defined as the per unit area exerted by the weight of the overlying air column in Earth's atmosphere on a given point. This pressure arises from the gravitational attraction pulling the atmospheric gases toward the planet's surface, resulting in a compressive that acts isotropically—equally in all directions—at any location due to the fluid properties of the air. Earth's atmosphere consists of a of gases, primarily and oxygen, that envelops the planet and behaves as a fluid under gravity. It extends outward to roughly 100 km above the surface, delineated by the as the conventional boundary with space, beyond which aerodynamic flight becomes impractical. However, approximately 99% of the atmosphere's total mass is contained below 50 km altitude, with the majority concentrated in the lower layers. The understanding of atmospheric pressure originated in 1643 when Italian physicist conducted experiments with a mercury-filled glass tube, inventing the first and demonstrating that the atmosphere's weight supports a column of mercury, thereby quantifying the pressure exerted by the air. At its core, this pressure can be expressed as P=FAP = \frac{F}{A}, where PP is the pressure, FF is the gravitational weight of the air column above the point, and AA is the horizontal cross-sectional area over which the force acts; it is commonly measured in pascals (Pa), with 1 Pa equivalent to 1 newton per square meter.

Units and Standards

Atmospheric pressure is quantified using various units, with the pascal (Pa) serving as the (SI) base unit, defined as one newton of force per square meter (1 Pa = 1 N/m²). In practice, multiples like the hectopascal (hPa), where 1 hPa equals 100 Pa, are widely used in and , as it aligns closely with typical pressure scales; the hectopascal is numerically equivalent to the millibar (mb). Traditional units derived from mercury barometry include the (inHg), prevalent in and some weather reporting, and the millimeter of mercury (mmHg), also called the , common in and laboratory contexts. Key conversions for standard sea-level pressure, which is defined as 1013.25 hPa (or 1013.25 mb), illustrate these relationships:
UnitValue for Standard Sea-Level Pressure
Hectopascal (hPa) or Millibar (mb)1013.25
(inHg)29.92
Millimeter of mercury (mmHg) or 760
The (ISA), established by the (ICAO), sets the reference sea-level pressure at exactly 1013.25 hPa under a temperature of 15°C to standardize performance calculations and atmospheric modeling. In meteorological applications, such as weather charts and forecasts from organizations like the (WMO), the hectopascal remains the preferred unit for its convenience in depicting pressure systems. Historically, the standard was defined as the average sea-level and is exactly 101325 Pa, providing a benchmark for non-SI contexts like chemistry and . Another legacy unit, the technical atmosphere (at), equals 98066.5 Pa and originated from practices relating to per square centimeter, though it is now largely obsolete in favor of SI units.

Physical Principles

Hydrostatic Equilibrium

Hydrostatic equilibrium in the atmosphere refers to the condition where the downward force of on a parcel of air is precisely balanced by the upward force arising from the vertical , resulting in no net of the . This balance maintains the static structure of the atmosphere, preventing wholesale collapse or expansion under gravitational influence. The gravitational force acts to compress air downward, while the , directed from high to low pressure regions, pushes air upward against . To derive the hydrostatic equation, consider a thin horizontal slab of air with cross-sectional area AA and infinitesimal thickness dzdz at altitude zz. The weight of this slab, acting downward, is ρgA dz\rho g A \, dz, where ρ\rho is the air density, gg is the acceleration due to gravity (approximately 9.8 m/s² near Earth's surface), and the slab's mass is ρA dz\rho A \, dz. This downward force is balanced by the difference in pressure forces across the slab: the upward force at the bottom is P(z)AP(z) A, and the downward force at the top is P(z+dz)AP(z + dz) A. In equilibrium, the net force is zero, so: P(z)A−P(z+dz)A=ρgA dz P(z) A - P(z + dz) A = \rho g A \, dz Dividing by A dzA \, dz and taking the limit as dz→0dz \to 0 yields the hydrostatic equation: dPdz=−ρg \frac{dP}{dz} = -\rho g Here, PP is atmospheric , and the negative sign indicates that pressure decreases with increasing altitude. This equation quantifies the rate at which pressure diminishes due to the weight of the overlying air. Integrating the hydrostatic equation provides a pressure profile. For a simplified isothermal atmosphere, where temperature TT is constant, the ideal gas law relates density to pressure as ρ=PMRT\rho = \frac{P M}{R T}, with MM the molar mass of air (approximately 0.029 kg/mol), and RR the universal gas constant (8.314 J/mol·K). Substituting into the hydrostatic equation gives: dPdz=−PMgRT \frac{dP}{dz} = -\frac{P M g}{R T} Separating variables and integrating from sea-level pressure P0P_0 at z=0z = 0 to P(z)P(z) at height zz yields the barometric formula: P(z)=P0exp⁡(−MgzRT) P(z) = P_0 \exp\left(-\frac{M g z}{R T}\right) This exponential decay describes how pressure falls with height in an isothermal layer, reflecting the decreasing weight of the air column above any given level. The role of gravity in hydrostatic equilibrium is fundamental: it imposes a vertical stratification where pressure at any altitude equals the integrated weight of the air column above, leading to a systematic decrease in pressure with height as the overlying mass diminishes. Near Earth's surface, gg varies slightly but is taken as constant for these derivations, emphasizing the compressive effect that shapes the atmosphere's density and pressure distribution.

Thermodynamic Basis

The thermodynamic basis of atmospheric pressure is rooted in the equation of state for air, which approximates the behavior of atmospheric gases as an ideal gas mixture. The ideal gas law relates pressure PP, mass density ρ\rho, and temperature TT through P=ρRTP = \rho R T, where RR is the specific gas constant for dry air, approximately 287 J/kg·K, derived from the universal gas constant divided by the molar mass of air (M≈0.029M \approx 0.029 kg/mol). This form links pressure directly to the density and thermal state of the air, enabling predictions of how variations in temperature and composition influence pressure gradients throughout the atmosphere. Temperature plays a central role in governing density and, consequently, pressure distribution. In warmer air, molecules gain kinetic energy and move farther apart, causing thermal expansion that decreases density at a constant pressure; conversely, cooler air contracts, increasing density. This density-temperature relationship affects the vertical structure of the atmosphere, particularly through the lapse rates. The dry adiabatic lapse rate, which describes the rate of temperature decrease with height for an unsaturated parcel of rising air under adiabatic conditions, is 9.8 °C/km. The average environmental lapse rate observed in the troposphere is about 6.5 °C/km. The composition of air also influences its density and pressure via the . Dry air, with an average of 28.97 g/mol, is denser than moist air because (molar mass 18 g/mol) displaces heavier nitrogen and oxygen molecules, slightly lowering the overall density and thus the pressure for a given temperature and height. This effect is small but measurable, in highly humid conditions compared to dry air. The atmospheric integrates the with the principle of to describe the full profile, where the vertical balances the weight of the overlying air column, modulated by variations from and composition. This combination yields the , relating height differences to and mean , essential for understanding large-scale atmospheric dynamics.

Standard Values

Mean Sea-Level Pressure

Mean sea-level (MSLP) refers to the standardized atmospheric exerted at the level of the surface under typical conditions, serving as a fundamental reference point in and . This value is defined as 1013.25 hectopascals (hPa), equivalent to 1013.25 millibars (mb), and corresponds to a of 15°C in the model. The standard MSLP is derived from extensive long-term observations of global atmospheric data, incorporating the effects of Earth's general circulation patterns to represent an idealized mean state. It is specifically calibrated for mid-latitudes around 45° north, where conditions balance various geophysical influences. In practice, actual MSLP exhibits minor latitudinal variations; for instance, it tends to be slightly lower near the —by about 1-2 hPa compared to polar regions—primarily due to the outward from , which reduces effective , combined with warmer temperatures that decrease air . This reference pressure forms the basis for altimetry calculations in , where aircraft instruments are calibrated to it, and for , enabling consistent analysis of pressure fields. Station pressure readings from land-based observatories are routinely reduced to MSLP using hydrostatic equations to normalize data across elevations and locations. The value was formally adopted by the (ICAO) in 1952 as part of the , with subsequent endorsements ensuring its widespread use.

Surface Pressure

Surface pressure denotes the atmospheric pressure exerted at Earth's ground level, typically varying between 980 and 1040 hectopascals (hPa), and is profoundly shaped by dynamic systems that cause short-term fluctuations. These systems drive the primary variability, with values often dipping below 1000 hPa during stormy conditions and rising above during fair , reflecting the weight of the overlying air column influenced by , , and motion. Key factors modulating surface pressure include high-pressure anticyclones and low-pressure cyclones. In anticyclones, sinking or subsiding air warms adiabatically, inhibiting formation and promoting clear skies with light winds and stable conditions. Conversely, cyclones feature converging surface winds that force air upward, cooling it and fostering development, , and turbulent storms. Additionally, a subtle diurnal cycle arises from solar heating, which expands air during the day and decreases pressure in the afternoon by about 1-1.5 hPa over land areas, resulting in pressure typically lowest around 4 a.m. and 4 p.m. and highest around 10 a.m. and 10 p.m. Globally, exhibits distinct patterns, with higher values in polar regions attributable to colder, denser air masses that enhance the weight of the atmosphere, and lower values in the due to intense solar heating that drives and reduces air . This latitudinal contrast forms the basis of large-scale circulation cells. The annual cycle amplifies these differences, particularly in winter when hemispheric gradients intensify, leading to stronger pressure contrasts between subtropical highs and subpolar lows. To ensure comparability across diverse elevations, meteorological stations routinely correct raw surface (station) pressure measurements to an equivalent sea-level value, accounting for the hypothetical air column from the site to using local profiles from the prior 12 hours. This adjustment facilitates consistent analysis of patterns, distinguishing observed surface variability from the fixed sea-level reference of 1013.25 hPa.

Variations

Altitude Effects

Atmospheric pressure decreases with increasing altitude due to the reduced weight of the air column above a given point, following principles of and the . In the , this decrease is approximately exponential, with pressure halving roughly every 5.5 km under standard conditions. The provides a for this variation in the , where temperature decreases linearly with height: P(z)=P0(1−γzT0)gMγRP(z) = P_0 \left(1 - \frac{\gamma z}{T_0}\right)^{\frac{g M}{\gamma R}} Here, P(z)P(z) is the pressure at geopotential altitude zz in meters, P0=101325P_0 = 101325 Pa is the sea-level pressure, γ=0.0065\gamma = 0.0065 K/m is the temperature lapse rate, T0=288.15T_0 = 288.15 K is the sea-level temperature, g=9.80665g = 9.80665 m/s² is the gravitational acceleration, M=0.0289644M = 0.0289644 kg/mol is the molar mass of dry air, and R=8.31432R = 8.31432 J/(mol·K) is the universal gas constant. This formula assumes a linear temperature profile and constant lapse rate, applicable up to the tropopause. In the U.S. Standard Atmosphere model, the troposphere extends from sea level to 11 km, where pressure drops to approximately 226 hPa. The lower stratosphere, from 11 km to 32 km, features isothermal conditions initially followed by gradual warming, with pressure continuing to decrease to about 8.7 hPa at 32 km. These altitude effects have practical implications, such as the onset of hypoxia effects for humans above approximately 3 km (10,000 feet) due to reduced partial pressure of oxygen, necessitating supplemental oxygen in aviation above this altitude during the day. Aircraft altimeters are calibrated using the standard sea-level pressure of 1013.25 hPa to indicate altitude accurately under standard conditions. Empirical observations align with the model, showing pressure around 75 hPa at 18 km, the typical cruising altitude for high-speed jet streams.

Horizontal and Local Variations

Atmospheric pressure displays notable horizontal variations across , driven by the large-scale circulation patterns of the atmosphere. In subtropical regions around 30° , descending air within the Hadley and Ferrel cells forms semi-permanent high-pressure belts. Conversely, subpolar lows near 60° arise from ascending air at the convergence of the Ferrel and polar cells. These latitudinal gradients, spanning thousands of kilometers, establish the primary zonal pressure distribution that influences global wind patterns. Local terrain features significantly modify horizontal pressure distributions by altering airflow dynamics. Mountain ranges act as barriers, forcing air to ascend on the windward side and often descend on the leeward side; however, in cases of strong blocking, this can induce cyclonic circulations and low-pressure development on the leeward side through lee cyclogenesis, where generation lowers pressures by several hPa relative to surrounding areas. Urban heat islands, characterized by elevated temperatures in built environments, generate local low-pressure zones due to and upward motion of heated air, drawing in cooler air through convergence and creating small pressure deficits compared to rural surroundings. Diurnal and seasonal cycles further contribute to horizontal pressure fluctuations on regional scales. During nighttime cooling, surface air contracts and densifies, increasing local by up to 1 hPa in calm conditions as radiative losses stabilize the . Seasonally, regimes produce pronounced shifts, with summer heating over continental interiors lowering pressures by 10–20 hPa relative to winter highs, enhancing cross-equatorial flows and reversing meridional gradients. These variations manifest across distinct spatial scales, differentiating synoptic from mesoscale processes. Synoptic-scale features, extending over approximately 1000 km, involve large systems such as extratropical cyclones and anticyclones that drive broad gradients over days. In contrast, mesoscale variations on 10–100 km scales arise from localized forcings, exemplified by sea breezes where daytime land heating establishes transient low-pressure over land, pulling marine air inland with gradients of 1–2 hPa over tens of kilometers.

Measurement Techniques

Historical Barometers

The invention of the mercury barometer in 1643 by Italian physicist marked the first reliable method for measuring atmospheric pressure. Torricelli filled a long glass tube, sealed at one end, with mercury and inverted it into a dish of the same liquid, observing that the mercury level dropped to leave a space above the column. At , this column typically stabilized at a height of 760 mm, with the height inversely proportional to atmospheric pressure—the greater the pressure, the taller the supported column. The underlying principle relies on , where atmospheric pressure balances the weight of the mercury column, expressed as P=ρghP = \rho g h, or rearranged to h=Pρgh = \frac{P}{\rho g}, with ρ\rho as the of mercury (13,600 kg/m³), gg as , and hh as column height. This setup demonstrated that air exerts a measurable downward force capable of supporting the fluid, refuting earlier notions of a perfect vacuum's impossibility. Refinements to the mercury barometer emerged in the early with the Fortin barometer, developed around by French instrument maker Jean Nicolas Fortin. This design incorporated an adjustable at the base, allowing precise leveling of the mercury surface to the scale's zero mark, which minimized reading errors from reservoir level variations and enhanced accuracy for scientific and meteorological use. The Fortin barometer laid groundwork for later non-liquid devices by emphasizing scale precision and portability. Early applications of barometers focused on weather prediction; in the 1660s, English scientist noted that sudden drops in mercury height foreshadowed severe storms, enabling rudimentary forecasts. By the , the Kew Observatory in became a global authority for instrument standardization, where Kew-pattern barometers—refined Fortin-style models—were tested and certified to ensure consistent measurements across observatories.

Modern Instruments and Mapping

Modern instruments for measuring atmospheric pressure have evolved to provide high precision, portability, and integration into global observation networks, enabling real-time data collection for weather forecasting and climate monitoring. Aneroid barometers, which rely on the deformation of a sealed metal diaphragm or capsule under varying pressure, serve as a foundational non-liquid alternative for direct measurements. The partially evacuated aneroid capsule expands or contracts in response to atmospheric pressure changes, with this mechanical deformation amplified through linkages to a calibrated dial or digital readout, typically scaled in hectopascals (hPa). These devices are widely used in aviation altimeters, where pressure readings are converted to altitude estimates assuming a standard atmosphere, aiding safe navigation by indicating height above mean sea level. Advancements in digital sensors have further enhanced measurement accuracy and automation in weather stations and portable systems. Capacitive transducers detect pressure variations through changes in the capacitance between a fixed electrode and a flexible diaphragm that deflects under pressure, while piezoresistive sensors measure strain-induced resistance changes in a silicon or metal diaphragm. Instruments like the PTB330 digital barometer employ these technologies to achieve accuracies of ±0.1 hPa at 20°C, with long-term stability of ±0.1 hPa per year, making them ideal for automated surface weather stations in remote or harsh environments. Similarly, Systems' Model 278 uses a capacitive ceramic sensor for barometric applications, offering dynamic response times of less than 100 ms and reliability in fluctuating conditions. Remote sensing techniques complement ground-based instruments by providing vertical and spatial profiles over vast areas. Radiosondes, launched via weather balloons, directly measure at multiple altitudes using integrated sensors as they ascend through the , yielding detailed profiles up to about 30-35 km with resolutions of 1 hPa or better. These balloon-borne systems transmit real-time data on , , and , forming a that supports models. Satellites such as the Geostationary Operational Environmental Satellites (GOES) infer surface and upper-level indirectly from cloud motion vectors and infrared imagery, tracking cloud patterns to estimate fields and gradients via geostrophic balance assumptions. These measurements feed into the creation of isobaric maps, which visualize atmospheric pressure distributions to highlight weather systems. Isobars, connecting points of equal pressure typically spaced every 4 hPa, reveal pressure gradients where closely spaced lines indicate steep changes, corresponding to stronger due to the relationship. models, such as the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecasting System, assimilate these observations to predict future pressure fields, generating high-resolution isobar charts that forecast phenomena like cyclones and anticyclones up to 10 days in advance. This integration of instrumental data and modeling ensures comprehensive mapping of pressure patterns, essential for , alerts, and analysis.

Alternative Methods

A barometer operates on the hydrostatic principle, where the height of the liquid column balances the atmospheric pressure, as originally conceptualized in Evangelista Torricelli's experiments in the 1640s, though he employed mercury for practicality. The formula governing this is h=Pρgh = \frac{P}{\rho g}, where hh is the column height, PP is atmospheric pressure, ρ\rho is fluid density, and gg is ; for with ρ≈1000 \kg/\m3\rho \approx 1000 \, \kg/\m^3, a standard pressure of 1 atm supports a column approximately 10.3 m tall. Early experiments, such as those by around 1660, demonstrated barometers using long tubes over 30 feet immersed in reservoirs, confirming pressure variations but highlighting their cumbersome scale compared to mercury devices. Despite their historical role, water barometers proved impractical for routine use due to water's low requiring excessively tall structures, which were vulnerable to and air dissolution—phenomena where dissolved gases migrate upward, distorting readings and necessitating frequent adjustments. Today, they find application primarily in educational settings to illustrate without the hazards of mercury. Alternative proxies for atmospheric pressure include indirect estimation methods in consumer devices. Altimeter watches often integrate GPS data to calibrate barometric sensors or infer pressure via standard atmospheric models, providing elevation-based approximations accurate to within 10 m vertically under clear conditions. Similarly, built-in barometers in smartphones enable crowdsourced , where aggregated readings from multiple devices pressure fields for mesoscale , improving forecast resolution in data-sparse regions. These approaches leverage widespread availability but depend on calibration to mitigate local influences on pressure-altitude relations.

Applications and Effects

Boiling Point Alteration

Atmospheric pressure directly influences the of liquids, as boiling occurs when the vapor pressure of the liquid equals the surrounding pressure. The Clausius-Clapeyron equation describes this relationship, quantifying how the natural logarithm of vapor pressure changes with : d(ln⁡P)dT=ΔHvapRT2\frac{d(\ln P)}{dT} = \frac{\Delta H_{\text{vap}}}{R T^2} where PP is the vapor pressure, TT is the in , ΔHvap\Delta H_{\text{vap}} is the , and RR is the . This equation illustrates that lower atmospheric pressure reduces the required for vapor pressure to reach equilibrium with the , thereby decreasing the . For , the standard is 100°C at sea-level of 1013 hPa (1 atm). At higher altitudes where is lower—such as 800 hPa at approximately 2 km elevation—the drops to about 93°C, and at 300 hPa around 9 km, it falls further to roughly 70°C. This reduction in has practical implications for cooking at high altitudes, where and other liquids reach at lower temperatures, leading to slower and requiring longer cooking times to achieve the same food . To counteract this, pressure cookers increase above atmospheric levels, raising the —for instance, to about 121°C at around 200 kPa total —and allowing faster cooking by enabling higher temperatures. The effect extends to other liquids, such as ethanol, which boils at 78°C at sea level but at a lower temperature in mountainous regions due to the decreased pressure.

Impacts on Weather and Aviation

Atmospheric pressure plays a fundamental role in shaping weather patterns through pressure gradients, which drive the movement of air masses and the formation of winds. In the absence of friction, horizontal pressure gradients achieve a balance with the Coriolis effect, known as geostrophic balance, where wind speed is determined by the formula vg=1ρfk×∇p\mathbf{v_g} = \frac{1}{\rho f} \mathbf{k} \times \nabla p Here, vg\mathbf{v_g} is the geostrophic wind velocity, ρ\rho is air density, ff is the Coriolis parameter (twice the vertical component of Earth's angular velocity), k\mathbf{k} is the unit vector in the vertical direction, and ∇p\nabla p is the horizontal pressure gradient. This balance results in winds flowing parallel to isobars, with low-pressure systems often associated with rising air, cloud formation, and fronts that bring precipitation and stormy conditions, while high-pressure systems promote sinking air and stable, clear weather. In aviation, accurate measurement and adjustment for local atmospheric pressure are critical for safe operations, particularly in determining aircraft altitude. Pilots set the altimeter to QNH, the observed pressure reduced to mean sea level using the standard atmosphere model, ensuring the instrument reads height above sea level during takeoff and landing. Above the transition altitude, typically around 3,000 to 18,000 feet depending on region, altimeters switch to the standard pressure setting of 1013.25 hPa (or 29.92 inHg), allowing consistent assignments across varying local pressures. Low atmospheric pressure reduces , which increases for a given , potentially affecting aircraft performance and requiring adjustments in climb rates and . Extreme low-pressure centers in storms, such as hurricanes, exemplify pressure's influence on , with typical central pressures around 950 hPa generating powerful winds and significant storm surges through the inverted . These low-pressure systems draw in moisture and amplify destructive potential, as lower central pressures correlate with higher surge heights due to the piling up of toward the storm's core. is projected to intensify such tropical cyclones, with models indicating potential increases in maximum wind speeds by 1 to 10% under 2°C global warming, leading to deeper low-pressure centers and heightened surge risks. Changes in barometric pressure also impact human health, particularly by triggering in susceptible individuals through mechanisms possibly involving serotonin imbalances or vascular responses to pressure shifts. Studies confirm that falling pressure, often preceding storms, is a common trigger, with 30% to 50% of migraine sufferers reporting weather-related episodes. In , pressure trends provide key indicators of impending changes; rising pressure signals improving conditions and fair weather, while falling pressure warns of approaching lows and unsettled , aiding in short-term predictions.

Extremes

Record Highs

The highest adjusted-to-sea-level atmospheric pressure ever recorded below 750 meters elevation is 1083.8 hPa, measured at Agata in the Russian Federation on 31 December 1968. This extreme value occurred within the , a semi-permanent winter anticyclone characterized by intense over snow-covered , which densifies the cold air mass and enhances surface pressure through large-scale . For locations above 750 meters elevation, the (WMO) recognizes 1089.1 hPa as the record, observed at Tosontsengel in on 30 December 2004. This measurement, taken at an elevation of 1,724.6 meters, also resulted from the expansion of the during a period of extreme winter cold, where clear skies and minimal promoted radiative cooling and air mass stabilization. The WMO verifies such records through rigorous evaluation, requiring data from calibrated mercury barometers or equivalent instruments compliant with international standards to ensure accuracy and eliminate elevation bias in sea-level adjustments. Other notable high-pressure extremes include readings around 1080 hPa associated with polar highs in , such as an earlier event at Agata in 1968 that approached the record threshold under similar anticyclonic conditions. These polar highs form primarily through in winter, where the loss of longwave radiation from the surface over vast continental areas creates dense, cold air domes that persist and intensify pressure gradients. Such record highs imply regions of exceptional atmospheric stability, with widespread inhibiting vertical motion and leading to prolonged clear skies and minimal . However, they often coincide with severe cold outbreaks, as the high-pressure systems export frigid air southward and eastward, influencing weather patterns across and beyond.

Record Lows

The lowest sea-level atmospheric pressure ever recorded, excluding tornadoes, is 870 hectopascals (hPa), measured at the center of on October 12, 1979, in the northwestern at coordinates 16°44'N, 137°46'E. This measurement was obtained by a United States Air Force WC-130 that penetrated the storm's eye, confirming the value through direct instrumentation and later validated by the (WMO) as the global record for intensity by central pressure. Tip also holds the distinction of being the largest on record, with a diameter exceeding 2,220 kilometers, driven by the extreme that fueled its massive scale. from the time corroborated the aircraft data, showing the storm's well-defined eye and expansive structure indicative of such low pressure. Other notable extremes include 872 hPa recorded in on October 23, 2015, in the eastern at 17°18'N, 105°47'W, marking the lowest pressure in the and verified by NOAA Hurricane Hunter aircraft dropsondes and estimates. Historically, prior to Tip, the lowest confirmed pressure in a was 875 hPa in June on August 17, 1975, in the western Pacific, as documented in joint typhoon warnings from the time. These records are maintained by the WMO, which relies on calibrated in-situ measurements and for verification to ensure accuracy amid the challenges of observing storm centers. In non-tropical systems like , local atmospheric pressures can plummet even further due to intense vortices, with measurements occasionally dropping to around 850 hPa or lower in extreme cases. For instance, a brief reading of 688 hPa was captured near the ground in the EF2 that struck , on April 21, 2007, representing a 194 hPa deficit from ambient conditions of approximately 882 hPa, as measured by a mobile vehicle equipped with a . Such localized lows are rarer and harder to verify due to instrument damage in high winds, but they highlight the microscale extremes possible in severe thunderstorms. These record lows have profound implications for storm dynamics and hazards. The steep pressure gradients associated with pressures below 900 hPa generate extreme winds often exceeding 250 kilometers per hour; Super , for example, sustained one-minute winds of 305 km/h (190 mph) at its peak. This intensity drives catastrophic storm surges, with Tip producing surges up to 12 meters along Japan's coast despite weakening before landfall, and contributes to widespread structural damage, heavy rainfall, and loss of life in affected regions.

References

  1. https://www.weather.gov/source/zhu/ZHU_Training_Page/winds/pressure_winds/Pressure.htm
  2. https://www.noaa.gov/jetstream/atmosphere/air-pressure
  3. https://scied.ucar.edu/learning-zone/how-weather-works/highs-and-lows-air-pressure
  4. https://projects.iq.harvard.edu/files/acmg/files/intro_atmo_chem_bookchap2.pdf
  5. https://pressbooks.uiowa.edu/clonedbook/chapter/pressure/
  6. https://science.nasa.gov/earth/earth-atmosphere/earths-atmosphere-a-multi-layered-cake/
  7. https://www.e-education.psu.edu/meteo3/l1_p3.html
  8. https://www.noaa.gov/jetstream/ll-pressure
  9. https://www.nist.gov/blogs/taking-measure/under-pressure-blaise-pascal-barometer-and-bike-tires
  10. https://www.engineeringtoolbox.com/standard-atmosphere-d_604.html
  11. https://eaglepubs.erau.edu/introductiontoaerospaceflightvehicles/chapter/international-standard-atmosphere-isa/
  12. https://www.sensorsone.com/atm-standard-atmosphere-pressure-unit/
  13. https://www.sensorsone.com/at-technical-atmosphere-pressure-unit/
  14. https://geo.libretexts.org/Bookshelves/Meteorology_and_Climate_Science/Practical_Meteorology_%28Stull%29/01%253A_Atmospheric_Basics/1.09%253A_Hydrostatic_Equilibrium
  15. https://farside.ph.utexas.edu/teaching/sm1/lectures/node54.html
  16. https://www.e-education.psu.edu/meteo300/node/7
  17. https://farside.ph.utexas.edu/teaching/336L/Fluidhtml/node186.html
  18. https://pubs.aip.org/aapt/ajp/article/65/5/404/1055036/On-the-barometric-formula
  19. http://www.atmo.arizona.edu/students/courselinks/fall11/atmo551a/ATMO_451a_551a_files/GasLawHydrostatic.pdf
  20. https://oyc.yale.edu/geology-and-geophysics/gg-140/lecture-3
  21. https://www.e-education.psu.edu/meteo300/node/644
  22. https://www.grc.nasa.gov/www/k-12/Summer_Training/FranktonES/Convection_main_page.html
  23. https://www.e-education.psu.edu/meteo300/node/594
  24. https://pressbooks-dev.oer.hawaii.edu/atmo/chapter/chapter-5-atmospheric-stability/
  25. https://www.ess.uci.edu/~yu/class/ess55/lecture.2.thermodynamics.pdf
  26. https://commons.erau.edu/cgi/viewcontent.cgi?article=1124&context=ijaaa
  27. https://www.faasafety.gov/files/events/NM/NM07/2023/NM07120280/FAA-P-8740-02-DensityAltitude.pdf
  28. https://atoc.colorado.edu/~cassano/atoc5050/Lecture_Notes/wh_ch3_part1.pdf
  29. https://www.ngdc.noaa.gov/stp/space-weather/online-publications/miscellaneous/us-standard-atmosphere-1976/us-standard-atmosphere_st76-1562_noaa.pdf
  30. https://www.tandfonline.com/doi/full/10.1080/23744731.2021.1949947
  31. https://skybrary.aero/articles/international-standard-atmosphere-isa
  32. https://ntrs.nasa.gov/api/citations/19630003300/downloads/19630003300.pdf
  33. https://blog.metservice.com/HectoPascal_Airpressure
  34. https://weather.metoffice.gov.uk/learn-about/weather/how-weather-works/high-and-low-pressure/weather-conditions
  35. https://www.noaa.gov/jetstream/global/global-atmospheric-circulations
  36. https://www.metoffice.gov.uk/hadobs/monitoring/climate_modes.html
  37. https://www.weather.gov/bou/pressure_definitions
  38. https://ntrs.nasa.gov/api/citations/19770009539/downloads/19770009539.pdf
  39. https://www.faa.gov/pilots/training/airman_education/topics_of_interest/hypoxia
  40. https://fiveable.me/introduction-climate-science/unit-4/global-atmospheric-circulation-patterns/study-guide/ZnR5iw1b4GEDjKXL
  41. https://resources.eumetrain.org/data/5/597/navmenu.php?tab=3&page=1.0.0
  42. https://www.coolgeography.co.uk/advanced/Urban_Climates.php
  43. https://patarnott.com/atms411/pdf/class2018/atmosTides.pdf
  44. https://pmc.ncbi.nlm.nih.gov/articles/PMC9124204/
  45. https://www.imdpune.gov.in/training/FTC1/Dynamic%2520Met/6.%2520Atmospheric%2520motion%2520scales%2520%281%29.pdf
  46. https://vortex.plymouth.edu/~stmiller/stmiller_content/Publications/Miller_etal_RoG_2003.pdf
  47. https://pmc.ncbi.nlm.nih.gov/articles/PMC3768090/
  48. https://openstax.org/books/college-physics/pages/11-6-gauge-pressure-absolute-pressure-and-pressure-measurement
  49. https://adsabs.harvard.edu/full/1944JRASC..38...41K
  50. https://www.rmg.co.uk/collections/objects/rmgc-object-42993
  51. https://www.weather.gov/media/zhu/ZHU_Training_Page/Met_Tutorials/Weather_Theory.pdf
  52. http://kirtlandflightcenter.org/wp-content/uploads/Weather_For_Aircrews.pdf
  53. https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1695&context=agronomyfacpub
  54. https://www.vaisala.com/en/products/weather-environmental-sensors/digital-barometer-ptb330
  55. https://www.setra.com/products/pressure/model-278-barometric-pressure-transducer
  56. https://www.noaa.gov/jetstream/upperair/radiosondes
  57. https://www.star.nesdis.noaa.gov/goesr/documents/ATBDs/Baseline/ATBD_GOES-R_Winds_v3.1_Feb2019.pdf
  58. https://www.noaa.gov/jetstream/upper-air-charts/constant-pressure-charts-thickness
  59. https://www.ecmwf.int/en/forecasts
  60. https://courses.lumenlearning.com/suny-albany-chemistry/chapter/gas-pressure-2/
  61. https://www.researchgate.net/publication/230735276_What_Is_Wrong_with_Water_Barometers
  62. https://digitalcommons.unomaha.edu/cgi/viewcontent.cgi?article=1012&context=chemfacpub
  63. https://www.suunto.com/Support/faq-articles/lifecycle/what-is-the-difference-between-a-gps-watch-and-a-gps-watch-with-barometer/
  64. https://journals.ametsoc.org/view/journals/wefo/32/2/waf-d-16-0135_1.xml
  65. https://rmets.onlinelibrary.wiley.com/doi/10.1002/met.1805
  66. https://chemed.chem.purdue.edu/genchem/topicreview/bp/ch14/clausius.php
  67. https://www.sciencedirect.com/topics/earth-and-planetary-sciences/boiling-point
  68. https://journals.ametsoc.org/view/journals/bams/98/7/bams-d-16-0174.1.xml
  69. https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=2144&context=extension_curall
  70. https://modernistcuisine.com/mc/how-pressure-cookers-work/
  71. https://www.engineeringtoolbox.com/boiling-points-fluids-gases-d_155.html
  72. https://atmos.uw.edu/academics/classes/2012Q1/211/Forces.pdf
  73. https://www.e-education.psu.edu/meteo101/l1_p9.html
  74. https://skybrary.aero/articles/altimeter-pressure-settings
  75. https://www.faa.gov/air_traffic/publications/atpubs/pcg_html/glossary-q.html
  76. https://www.aoml.noaa.gov/hrd-faq/
  77. https://www.gfdl.noaa.gov/global-warming-and-hurricanes/
  78. https://pmc.ncbi.nlm.nih.gov/articles/PMC10940451/
  79. https://news.cuanschutz.edu/news-stories/migraines-can-be-debilitating-and-frustrating-when-triggered-by-weather-you-cant-control
  80. https://science.howstuffworks.com/nature/climate-weather/atmospheric/barometer.htm
  81. https://wmo.int/sites/default/files/2024-07/Table_Records_02Jul2024.pdf
  82. https://journals.ametsoc.org/view/journals/mwre/132/8/1520-0493_2004_132_1952_pwtmsl_2.0.co_2.xml
  83. https://www.atmos.albany.edu/daes/atmclasses/atm305/Jones-and-Cohen-2011.pdf
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