The abbreviation atm stands for standard atmosphere, a non-SI, non-metric, but very convenient unit of pressure.
1 atm is about the air pressure at sea level and 0 ºC. It is currently defined as exactly 101,325 Pa (the pascal is the SI unit of pressure).
The standard atmosphere is an important unit because it expresses pressure in intuitive terms. 0.5 atm is half the air pressure you feel now, and 2 atm is twice the air pressure you feel now.
Additionally, 1 atm is useful in defining Standard Pressure and Temperature (STP). Although IUPAC currently defines the pressure in STP as 100 kPa (about 1% less than 1 atm), other committees still use 1 atm for STP.
The unit atm is especially used when the precise air pressure is unimportant. For example, I might say that I heated my sample in dry air at 1 atm. In this case I mean that I didn’t measure the air pressure, but I assumed it was basically the pressure of the room, which is more or less 1 atm.
Atmospheres might also be used in scientific fields where the reference to the real atmosphere is important, or when trying to communicate to the general public.
The downsides of the standard atmosphere are that it is not Si, not metric, and relatively coarse.
There is also a unit “total atmospheres” (ata) which is used specifically when dealing with the pressure from multiple fluids. For example, under water you have water pressure and air pressure; a maverick scientist who decries the SI system may want to write the water pressure in terms of ata to include the air pressure above. In this case, 4 ata means that the water has a pressure of 3 atm (because there is 1 atm from the air above).
| Pros | Cons |
| – expresses pressure in intuitive terms – easy reference to the real atmosphere – easily communicates to the general public | – it is not SI – not metric – relatively coarse |
This article was a definition of atm. If you are interested in all the different, strange units that are used to measure pressure, I wrote an entire article explaining what each unit is, and how to convert between them. Or, if there is a unit you are especially interested in, you can check out one of the other unit definitions I’ve written.
Other Pressure Measurements:
Pa
mbar or bar
Torr
at
psi or ksi