Standard atmosphere (unit) facts for kids

The standard atmosphere (symbol: atm) is a special unit used to measure pressure. Think of pressure as the force that air or water pushes down with. One standard atmosphere is exactly 101325 Pascals (Pa).

This unit is often used as a "reference pressure" or "standard pressure." It's very close to the average air pressure you'd feel at sea level on Earth.

How the Standard Atmosphere Was Defined

Long ago, the standard atmosphere was first set based on how much pressure a column of mercury would create. Imagine a tube filled with mercury that is 760 millimeters (about 30 inches) tall. The pressure from that mercury at a certain temperature (0 degrees Celsius) was called one standard atmosphere.

This measurement was important for science. It helped scientists have a common way to talk about pressure. For example, the boiling point of water (100 degrees Celsius) was defined at this specific pressure.

In 1954, a big meeting called the 10th General Conference on Weights and Measures decided to make the definition even more precise. They said one standard atmosphere is exactly 101325 Pascals. This new definition made the standard atmosphere independent of any specific substance like mercury. It made it a more universal and exact measurement.

Different Ways to Measure Pressure

One standard atmosphere (1 atm) can be described using many different units of pressure. This is because different fields and countries use various ways to measure the same thing.

Here are some ways to think about 1 atm:

• It's exactly 101325 Pascals (Pa). Pascals are the main units for pressure in science.
• It's about 1.01325 bar. A bar is another unit often used in meteorology (weather science).
• It's roughly 10.33 meters of water (m H₂O). This means the pressure from a column of water about 10 meters tall is similar to 1 atm.
• It's exactly 760 torr (Torr). This unit is named after Evangelista Torricelli, who invented the barometer.
• It's about 14.6959 pounds-force per square inch (lbf/in² or psi). This unit is commonly used in the United States, for example, to measure tire pressure.

Understanding 'ata'

You might also see the unit ata, which stands for "atmosphere absolute." This unit is used to show the total pressure in a system, especially when compared to a perfect vacuum (zero pressure).

For example, if you are underwater, the pressure you feel includes the air pressure from above the water (1 atm) plus the pressure from the water itself. So, if the total pressure is 3 ata, it means 1 atm is from the air, and the other 2 atm are from the water pushing down.

See also

In Spanish: Atmósfera (unidad) para niños