atmospheric pressure

How atmospheric pressure differs at different altitudesNear the surface of the Earth, atmospheric pressure decreases almost linearly with increasing altitude. At higher altitudes, pressure decreases exponentially. A large commercial airliner typically has a cruising altitude 10–13 km (6–8 miles) above Earth's surface. Because atmospheric pressure is much lower at that altitude than at sea level, the airliner needs a pressurized cabin to enable people onboard to breathe comfortably. An airliner maintains an interior pressure approximating air pressure at sea level (1,013.25 mbar, or one standard atmosphere) while the atmospheric pressure at outside the plane, as shown by the dashed yellow line, can be less than 200 mbar—an environment hostile to humans.

atmospheric pressure, force per unit area exerted by an atmospheric column (that is, the entire body of air above the specified area).

Atmospheric pressure can be measured with a mercury barometer (hence the commonly used synonym barometric pressure), which indicates the height of a column of mercury that exactly balances the weight of the column of atmosphere over the barometer. Atmospheric pressure is also measured using an aneroid barometer, in which the sensing element is one or more hollow, partially evacuated, corrugated metal disks supported against collapse by an inside or outside spring; the change in the shape of the disk with changing pressure can be recorded using a pen arm and a clock-driven revolving drum.

Atmospheric pressure is expressed in several different systems of units: millimeters (or inches) of mercury, pounds per square inch (psi), dynes per square centimeter, millibars (mb), standard atmospheres, or kilopascals.

Standard sea-level pressure, by definition, equals each of the following:

  • 760 mm (29.92 inches) of mercury
  • 14.70 pounds per square inch
  • 1,013.25 × 103 dynes per square centimeter
  • 1,013.25 millibars
  • one standard atmosphere
  • 101.325 kilopascals

These values show only very small variations. For example, the highest and lowest sea-level pressures ever recorded are 32.01 inches (in the middle of Siberia) and 25.90 inches (in a typhoon in the South Pacific). The small variations in pressure that do exist largely determine the wind and storm patterns of Earth.

January average atmospheric temperature
2 of 3
January average atmospheric temperatureMap of global mean air pressure at sea level in January, where high-pressure centers are more diffuse. Central Asia, however, is dominated by a strong high-pressure cell.
July average atmospheric temperature
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July average atmospheric temperatureMap of global mean air pressure at sea level in July, where high-pressure centers are more pronounced, especially over the North Atlantic and North Pacific oceans.

Near Earth’s surface, atmospheric pressure decreases with height at a rate of about 3.5 millibars for every 30 meters (100 feet). However, over cold air the decrease in pressure can be much steeper because its density is greater than warmer air. The pressure at 270,000 meters (10−6 mb) is comparable to that in the best human-made vacuum ever attained. At heights above 1,500 to 3,000 meters (5,000 to 10,000 feet), the pressure is low enough to produce mountain sickness and severe physiological problems unless careful acclimatization is undertaken.

The Editors of Encyclopaedia Britannica This article was most recently revised and updated by J.E. Luebering.
Citation Information
Article Title: atmospheric pressure
Website Name: Encyclopaedia Britannica
Publisher: Encyclopaedia Britannica, Inc.
Date Published: 30 August 2024
URL: https://www.britannica.comhttps://www.britannica.com/science/atmospheric-pressure
Access Date: November 21, 2024