What Is Standard Atmosphere (atm)?
The standard atmosphere, written as atm, is a unit used to measure pressure. Pressure tells us how much force is pushing on an area. One standard atmosphere is based on the average air pressure at sea level on Earth under normal conditions.
Scientists and engineers often use atm when they talk about gases, weather, or pressure in containers. It is not an official SI unit, but it is still very common in chemistry and physics.
Definition
The standard atmosphere is defined as exactly:
- 1 atm = 101325 pascals (Pa)
The pascal (Pa) is the SI unit of pressure. Because this number is large, people also use other equal forms:
- 1 atm = 101.325 kilopascals (kPa)
- 1 atm = 1.01325 bar
- 1 atm = 1013.25 hectopascals (hPa)
- 1 atm ≈ 760 millimeters of mercury (mmHg)
- 1 atm ≈ 14.696 pounds per square inch (psi)
So when you see atm, you can think of it as a standard, fixed level of pressure that matches these other values.
History / Origin
People first became interested in air pressure in the 1600s. Around 1643, Evangelista Torricelli, an Italian scientist, built a device using a glass tube and liquid mercury. He found that air pressure could hold up a column of mercury in the tube.
Over time, scientists noticed that normal air at sea level could hold up a mercury column about 760 millimeters high. This became a useful reference. For many years, one atmosphere was thought of as the pressure that supports a 760 millimeter tall column of mercury at 0 degrees Celsius under standard gravity.
In 1954, the International Union of Pure and Applied Chemistry (IUPAC) gave a clear, exact definition. They set one standard atmosphere equal to 101325 pascals. This linked atm directly to the SI system while keeping it close to the old mercury column idea.
Symbol & Abbreviation
The main symbol for standard atmosphere is:
- atm
Important notes.
- atm means standard atmosphere.
- It is different from at, which stands for technical atmosphere, an older unit that is not widely used now.
- When you see a number like 2 atm, it means the pressure is two times the standard atmosphere.
Current Use Around the World
Even though atm is not an SI unit, it is still used in many areas:
- Chemistry. Gas laws, such as the ideal gas law, often use pressure in atm. For example, a problem might say a gas is at 1.0 atm.
- Physics and engineering. Some textbooks and diagrams use atm to show pressures compared to normal air pressure.
- Diving and underwater work. Scuba divers often think in atmospheres. About every 10 meters of depth in seawater adds roughly 1 atm of pressure.
- Medicine. Hyperbaric oxygen chambers sometimes describe treatment pressure in atm, such as 2 atm.
However, many official scientific documents prefer SI units, such as pascal (Pa) and kilopascal (kPa). Weather reports more often use hectopascal (hPa) or millibar, which have values close to atm but are more standard today.
Example Conversions
Here are some simple conversions using 1 standard atmosphere.
From atm to other units
| Pressure | Equals |
|---|---|
| 1 atm | 101325 Pa |
| 1 atm | 101.325 kPa |
| 1 atm | 1.01325 bar |
| 1 atm | 1013.25 hPa |
| 1 atm | ≈ 760 mmHg |
| 1 atm | ≈ 14.7 psi |
From other units to atm
| Pressure | In atmospheres |
|---|---|
| 101325 Pa | 1 atm |
| 202650 Pa | 2 atm |
| 50 kPa | ≈ 0.49 atm |
| 2 bar | ≈ 1.97 atm |
| 380 mmHg | ≈ 0.5 atm |
| 30 psi | ≈ 2.04 atm |
To convert between units, you multiply or divide by the correct factor. For example.
- To change atm to kPa, multiply by 101.325.
- To change kPa to atm, divide by 101.325.
Related Units
Many other pressure units are related to the standard atmosphere.
- Pascal (Pa). The SI unit of pressure. 1 atm = 101325 Pa.
- Kilopascal (kPa). 1 kPa = 1000 Pa. 1 atm ≈ 101.325 kPa.
- Bar. A common scientific unit. 1 bar = 100000 Pa. 1 atm ≈ 1.01325 bar.
- Hectopascal (hPa). Used in weather reports. 1 hPa = 100 Pa. 1 atm = 1013.25 hPa.
- Millibar (mbar). 1 mbar = 1 hPa. So 1 atm = 1013.25 mbar.
- Millimeter of mercury (mmHg). Based on a mercury column. 1 atm ≈ 760 mmHg.
- Pound per square inch (psi). Common in car tire pressure. 1 atm ≈ 14.7 psi.
- Technical atmosphere (at). An old unit based on 1 kilogram force per square centimeter. It is close to but not the same as 1 atm.
FAQs
What does 1 atm actually represent?
One atm is the pressure of Earths air at sea level under normal conditions. It is exactly 101325 pascals and is used as a standard reference pressure.
Is atm an SI unit?
No. Atm is not an official SI unit. The SI unit for pressure is the pascal (Pa). However, atm is still allowed and often used in chemistry and physics because it is very convenient.
Why do chemists like to use atm?
Chemists use atm because many gas laws were first written using this unit. At normal lab conditions, pressures are often close to 1 atm, which makes calculations and thinking about real gases easier.
How many atm is normal air pressure?
Normal air pressure at sea level is defined as 1 atm. In real life it changes a little with weather, but it stays close to this value for most places at sea level.
How many atm are underwater?
At the surface of the ocean, the pressure is about 1 atm from the air. For every 10 meters of depth in seawater, the pressure adds about 1 extra atm. So at 20 meters deep, the total pressure is about 3 atm, one from air and two from water.
What is the difference between atm and bar?
Both are pressure units, and they are close but not equal. 1 bar is exactly 100000 Pa, while 1 atm is 101325 Pa. So 1 atm is about 1.01325 bar.
Can I use atm in engineering work?
Yes, you can, but many engineering standards prefer kPa, MPa, or bar. When writing reports or designs, it is best to check which unit your field or company requires and convert from atm if needed.