Atmospheric pressure

Atmospheric pressure , also known as air pressure or barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard…

Atmospheric pressure, alternatively termed air pressure or barometric pressure (named after the barometer), represents the force exerted by the Earth's atmosphere. The standard atmosphere (symbol: atm) is a recognized unit of pressure, precisely defined as 101,325 Pa (1,013.25 hPa). This value is equivalent to 1,013.25 millibars, 760torr (approximately 760mmHg), about 29.9212inHg, or roughly 14.696psi. The atm unit closely approximates the average atmospheric pressure observed at mean sea level on Earth.

Under typical conditions, atmospheric pressure can be accurately estimated by the hydrostatic pressure generated by the weight of the air column situated above a specific measurement location. Consequently, atmospheric pressure diminishes with increasing elevation due to a reduction in the overlying atmospheric mass. Given the atmosphere's relative thinness compared to Earth's radius, particularly its denser lower layers, Earth's gravitational acceleration can be considered nearly constant with altitude, thus having minimal impact on this pressure decrease. Pressure is quantified as force per unit area, with the SI unit being the pascal (1 pascal = 1 newton per square meter, 1N/m²). An average column of air, possessing a cross-sectional area of 1 square centimeter (cm§4^{5§) and extending from mean sea level to the upper reaches of Earth's atmosphere, has an approximate mass of 1.03 kilograms. This column exerts a force, or "weight," of about 10.1 newtons, yielding a pressure of 10.1 N/cm§6^{7§ or 101kN/m§10^{11§ (101 kilopascals, kPa). Similarly, an air column with a cross-sectional area of 1in§14^{15§ would exert a weight of approximately 14.7lbf, resulting in a pressure of 14.7lbf/in§20^21§.}}}}

Mechanism

Atmospheric pressure originates from the planet's gravitational pull on the atmospheric gases situated above its surface. This pressure is determined by several factors: the planet's mass, the surface radius, and the quantity, composition, and vertical distribution of the atmospheric gases. Furthermore, it is influenced by planetary rotation and localized phenomena, including wind velocity, temperature-induced density fluctuations, and compositional variations.

Mean Sea-Level Pressure

The mean sea-level pressure (MSLP) refers to the atmospheric pressure measured at mean sea level. This specific pressure value is routinely disseminated in weather forecasts by meteorologists across various media, including radio, television, newspapers, and online platforms.

In aviation, the altimeter setting constitutes a crucial adjustment based on atmospheric pressure.

The average sea-level pressure is recorded as 1,013.25 hPa (29.921 inHg; 760.00 mmHg). Within aviation weather reports (METAR), the QNH value is globally broadcast in hectopascals or millibars (where 1 hectopascal equals 1 millibar). Conversely, in the United States, Canada, and Japan, altimeter settings are communicated in inches of mercury, typically to two decimal places. The United States and Canada additionally provide sea-level pressure (SLP) in the remarks section of their weather codes, adjusted to sea level using an alternative methodology, and expressed in hectopascals or millibars, distinct from the internationally transmitted code. Notably, Canadian public weather reports present sea level pressure in kilopascals. In the US weather code remarks, only the final three digits are transmitted, with decimal points and the most significant digits omitted. For instance, 1,013.2 hPa (14.695 psi) is transmitted as 132; 1,000 hPa (100 kPa) as 000; and 998.7hPa as 987. This three-digit transmission system implies that the same code (e.g., 800) would represent both 1080.0 hPa and 980.0 hPa.

The Earth's highest recorded sea-level pressure is observed in Siberia, where the Siberian High frequently exceeds 1,050 hPa (15.2 psi; 31 inHg), with historical peaks approaching 1,085 hPa (15.74 psi; 32.0 inHg). Conversely, the lowest measurable sea-level pressure occurs within the cores of tropical cyclones and tornadoes, reaching a documented minimum of 870 hPa (12.6 psi; 26 inHg).

Surface Pressure

Surface pressure is defined as the atmospheric pressure measured at a specific point on Earth's surface, encompassing both landmasses and oceans. This pressure exhibits a direct proportionality to the total mass of air situated above that particular location.

For computational efficiency, atmospheric models, including general circulation models (GCMs), typically forecast the nondimensional logarithm of surface pressure.

The Earth's average surface pressure measures 985 hPa. This differs from mean sea-level pressure, which is derived by extrapolating pressure values to sea level for all locations, regardless of their actual elevation. Within the International Standard Atmosphere (ISA), the average pressure at mean sea level (MSL) is defined as 1,013.25 hPa, equivalent to 1 atmosphere (atm) or 29.92 inches of mercury.

Pressure (P) is fundamentally linked to force (F) and area (A) by the formula P = F/A. Since force can be expressed as mass (m) multiplied by acceleration due to gravity (g), the relationship becomes P = (m*g)/A. Consequently, atmospheric pressure directly correlates with the weight per unit area of the atmospheric column situated above a specific point.

Variation with Altitude

Atmospheric pressure on Earth exhibits variation with surface altitude; consequently, mountainous regions typically experience lower air pressure compared to sea level. This pressure gradient progresses smoothly from the Earth's surface through to the mesosphere's upper boundary. While meteorological conditions induce pressure fluctuations, NASA has compiled averaged global conditions throughout the year. A fundamental principle is that atmospheric pressure diminishes as altitude increases, allowing for the calculation of pressure at specific elevations. Furthermore, temperature and humidity exert influence on atmospheric pressure. Pressure is directly proportional to temperature and inversely proportional to humidity, with both factors being crucial for precise calculations. The accompanying graph above was formulated based on a temperature of 15 °C and a relative humidity of 0%.

At lower altitudes above sea level, atmospheric pressure typically declines by approximately 1.2 kPa (12 hPa) for each 100-meter ascent. For greater elevations within the troposphere, the barometric formula provides a relationship between atmospheric pressure p and altitude h, expressed by the subsequent equation:

${\begin{aligned}p&=p_{0}\cdot \left(1+{\frac {L\cdot h}{T_{0}}}\right)^{-{\frac {g\cdot M}{R_{0}\cdot L}}}\\&=p_{0}\cdot \left(1+{\frac {g\cdot h}{c_{\text{p}}\cdot T_{0}}}\right)^{-{\frac {c_{\text{p}}\cdot M}{R_{0}}}}\end{aligned}}$ 37§ + L ⋅ h T §5556§ ) − g ⋅ M R §8586§ ⋅ L = p §113114§ ⋅ ( §125126§ + g ⋅ h c p ⋅ T §156157§ ) − c p ⋅ M R §192193§ {\displaystyle {\begin{aligned}p&=p_{0}\cdot \left(1+{\frac {L\cdot h}{T_{0}}}\right)^{-{\frac {g\cdot M}{R_{0}\cdot L}}}\\&=p_{0}\cdot \left(1+{\frac {g\cdot h}{c_{\text{p}}\cdot T_{0}}}\right)^{-{\frac {c_{\text{p}}\cdot M}{R_{0}}}}\end{aligned}}}

The parameters within these equations include:

Localized Atmospheric Pressure Fluctuations

Atmospheric pressure exhibits substantial global variability, and these pressure differentials are critical for meteorological and climatological analyses. Certain pressure fluctuations demonstrate remarkable regularity. A significant contributor to these variations is the phenomenon of atmospheric tides, which manifest most intensely in tropical regions, characterized by amplitudes reaching several hectopascals, while being nearly absent in polar latitudes. These tropical tidal pressure variations are primarily composed of two superimposed harmonic components: a circadian (24-hour) cycle and a semi-circadian (12-hour) cycle.

Recorded Extremes

The maximum barometric pressure, adjusted to sea level, ever documented at an elevation exceeding 750 meters was 1,084.8 hPa (32.03 inHg; 1.0706 atm), recorded in Tosontsengel, Mongolia, on December 19, 2001. Conversely, the highest sea-level-adjusted barometric pressure observed below 750 meters occurred in Agata, Evenk Autonomous Okrug, Russia (66°53'N, 93°28'E, elevation: 261 m, 856 ft), registering 1,083.8 hPa (32.005 inHg) on December 31, 1968. This disparity arises from a problematic assumption, specifically the use of a standard lapse rate, inherent in normalizing high-elevation pressure measurements to standard sea level. As altitude increases, the accuracy of this conversion diminishes, leading to the establishment of an arbitrary 750-meter threshold for ensuring adequate precision in record-keeping.

The Dead Sea, recognized as the lowest terrestrial elevation at 430 meters (1,410 ft) below sea level, consequently exhibits a characteristically elevated atmospheric pressure, typically around 1,065hPa. A record for surface pressure below sea level, measuring 1,081.8 hPa (31.95 inHg; 1.0677 atm), was established on February 21, 1961.

The lowest atmospheric pressure recorded outside of a tornadic event was 870 hPa (26 inHg; 0.86 atm), observed on October 12, 1979, within Typhoon Tip in the western Pacific Ocean. This measurement was derived from an instrumental observation conducted by a reconnaissance aircraft.

Pressure Measurement via Water Column Depth

One standard atmosphere (101.325 kPa or 14.7 psi) corresponds to the pressure exerted by a column of freshwater approximately 10.3 meters (33.8 ft) in height. Consequently, a diver submerged 10.3 meters experiences a total pressure of approximately two atmospheres, comprising one atmosphere from the air and one from the water column. Conversely, under standard atmospheric conditions, 10.3 meters represents the theoretical maximum height to which water can be elevated through suction.

Low-pressure systems, such as natural gas lines, are occasionally quantified in inches of water, commonly denoted as w.c. (water column) gauge or w.g. (inches water) gauge. In the United States, a standard residential gas appliance is typically rated for a maximum pressure of §56§⁄§78§ psi (3.4 kPa; 34 mbar), which approximates 14 inches of water gauge (w.g.). Analogous metric units, employing various nomenclature and notations based on millimeters, centimeters, or meters, have become less prevalent in contemporary usage.

Influence of Pressure on Liquid Boiling Points

Under Earth's standard atmospheric pressure, pure water boils at 100 °C (212 °F). The boiling point is defined as the temperature at which a liquid's vapor pressure equilibrates with the ambient atmospheric pressure. Consequently, the boiling point of liquids decreases with reduced pressure and increases with elevated pressure. Therefore, culinary practices at high altitudes necessitate modifications to recipes or the employment of pressure cooking techniques. A rudimentary estimation of elevation can be achieved by determining the boiling temperature of water, a method historically employed by explorers during the mid-19th century. Conversely, to facilitate the evaporation of a liquid at a lower temperature, such as in distillation processes, atmospheric pressure can be reduced through the application of a vacuum pump, exemplified by its use in a rotary evaporator.

Barometric Measurement in Cartography and Surveying

A significant application of the understanding that atmospheric pressure correlates directly with altitude emerged in the determination of topographical elevations, facilitated by advancements in reliable pressure measurement instrumentation. In 1774, Nevil Maskelyne undertook the confirmation of Newton's gravitational theory on Schiehallion mountain in Scotland, necessitating precise elevation measurements across the mountain's flanks. This endeavor is historically recognized as the Schiehallion experiment. William Roy subsequently validated Maskelyne's height determinations using barometric pressure, achieving an agreement within one meter (3.28 feet). This methodology proved, and remains, invaluable for surveying and cartographic applications.

References

Contemporary Global Mean Sea-Level Pressure Map

Current map of global mean sea-level pressure Archived 2017-02-08 at the Wayback Machine
NASA's 1976 Standard Atmosphere Model
Source code and equations for the 1976 Standard Atmosphere.
A mathematical model of the 1976 U.S. Standard Atmosphere.
Calculator utilizing multiple units and properties for the 1976 Standard Atmosphere.
Calculator providing standard air pressure at a specified altitude, or the corresponding altitude for a standard pressure.
Method for calculating pressure from altitude and vice versa.

Experiments.

Videos demonstrating atmospheric pressure experiments from Georgia State University's HyperPhysics.
Demonstration illustrating a can's collapse after boiling water internally and subsequent immersion in ice-cold water.

Atmospheric pressure