Atmosphere (unit)

Definition

The standard atmosphere is defined as exactly 101325 pascals (Pa). Its symbol is atm. Historically it was tied to the height of a mercury column, but the modern definition fixes the value in terms of the SI unit pascal. For background on pressure concepts see atmospheric pressure and for the geographic reference see sea level.

History and development

Early practice measured pressure with mercury barometers and identified average sea-level pressure with the column height of mercury (commonly quoted as 760 millimetres of mercury). Over time international agreements replaced that empirical notion with an exact numeric value expressed in pascals. With the spread of the SI, agencies encouraged use of the pascal; the pascal thus became the preferred unit for formal reporting, while the atmosphere persisted as a familiar comparative unit.

Conversions and equivalents

• 1 atm = 101325 Pa (exact by definition)
• 1 atm = 760 Torr or 760 mmHg (by historical definition)
• 1 atm ≈ 1.01325 bar (the bar = 100000 Pa; see bar)
• 1 atm ≈ 1013.25 hPa (hectopascals, commonly used in meteorology)
• 1 atm ≈ 14.7 psi (pounds per square inch)

Uses and notable facts

The atmosphere is a convenient unit for expressing pressures encountered in daily life and in laboratory settings: tire pressures, scuba diving limits, gas-supply pressures and standard-state conditions in chemistry are often compared to 1 atm. Modern metrology and SI-compliant reporting favor pascals or multiples such as the kilopascal or hectopascal, but the atmosphere remains useful as an intuitive baseline: for example, "vacuum of 0.1 atm" or "pressure of 2 atm" quickly convey relative magnitudes without numeric conversions.

When precision and SI conformity are required, convert atmospheres to pascals or another SI-accepted unit rather than using atm as the primary measurement unit.