What is atmosphere?
An atmosphere (atm) is a pressure unit equal to mean atmospheric pressure at sea level, approximately 101,325 pascals. It is used as a reference in chemistry, diving tables, and altitude calculations.
Real-world uses
Atmospheres are used as a reference pressure in chemistry (standard conditions: 1 atm, 25°C), scuba diving (each 10 m of seawater adds about 1 atm), and high-pressure industrial applications like autoclaves and pressure vessels.
History
The standard atmosphere was originally defined as the pressure exerted by a 760 mm column of mercury at 0°C under standard gravity. Evangelista Torricelli first measured atmospheric pressure in 1644 using a mercury barometer, establishing this foundational concept.
Common mistakes
Confusing standard atmosphere (101,325 Pa) with technical atmosphere (1 kgf/cm² ≈ 98,066.5 Pa). Also, assuming atmospheric pressure is constant at 1 atm everywhere—it varies with altitude, weather, and temperature.
What is millibar?
A millibar is a pressure unit equal to one thousandth of a bar. It is the standard unit for atmospheric pressure in meteorology and weather forecasting, particularly in aviation and marine contexts.
Real-world uses
Millibars are the traditional unit in meteorology for reporting atmospheric pressure, particularly in weather maps and hurricane tracking. Standard atmospheric pressure is 1013.25 mbar, and tropical cyclones can have central pressures below 900 mbar.
History
The millibar was introduced by Sir Napier Shaw in 1909 for meteorological use. It dominated weather reporting throughout the 20th century. Since 1982, the WMO has officially recommended hectopascals instead, though both represent exactly 100 Pa.
Common mistakes
Not realizing that 1 millibar = 1 hectopascal exactly. Meteorological services have largely switched to hPa, but the numerical values are identical. Also, assuming lower millibar values mean calmer weather—lower pressure often indicates storms.
When is this conversion used?
Tyre pressure gauges, HVAC systems, and industrial equipment may use different pressure units depending on the manufacturer and region. Converting between psi, bar, and kPa is common in automotive and engineering work.
Worked examples
1 atmosphere = 1,013.25 millibar
1 millibar = 100 pascal
How to convert atmosphere to millibar
To convert atmosphere to millibar, multiply the value by 1,013.25.
To convert millibar back to atmosphere, multiply by 0.00098692.
Measurement standards
The pascal (Pa) is the SI derived unit of pressure, defined as one newton per square metre. The World Meteorological Organization mandates the hectopascal (hPa) for atmospheric pressure reporting in aviation and weather services.
Did you know?
The pressure at the bottom of the Mariana Trench, nearly 11 km deep, exceeds 1,086 bar — over a thousand times atmospheric pressure. At this depth, water is compressed by about 5%, making it measurably denser than at the surface.
Quick reference: atmosphere to millibar
| atmosphere | millibar |
|---|---|
| 0.1 | 101.325 |
| 0.5 | 506.625 |
| 1 | 1,013.25 |
| 2 | 2,026.5 |
| 5 | 5,066.25 |
| 10 | 10,132.5 |
| 25 | 25,331.25 |
| 50 | 50,662.5 |
| 100 | 101,325 |
| 250 | 253,312.5 |
| 500 | 506,625 |
| 1,000 | 1,013,250 |
Common values
| atmosphere | millibar | |
|---|---|---|
| Car tyre | 2.17123119 atmosphere | 2,200 millibar |
| Standard atmosphere | 1 atmosphere | 1,013.25 millibar |
| Blood pressure (systolic) | 0.15790772 atmosphere | 160 millibar |
| Deep-sea submersible | 1,085.61559339 atmosphere | 1,100,000 millibar |
| Bicycle tyre | 6.11892425 atmosphere | 6,200 millibar |