Atmospheric pressure, also known as barometric pressure, represents the force exerted by the weight of air molecules in Earth's atmosphere against any surface. This fundamental physical phenomenon occurs because air has mass, and the entire column of air above any point on Earth's surface creates a downward force due to gravity. The concept was first scientifically demonstrated in 1643 by Italian physicist Evangelista Torricelli, who invented the mercury barometer and proved that air has weight.
At sea level under standard conditions, atmospheric pressure equals approximately 1013.25 hectopascals (hPa) or 14.7 pounds per square inch (psi). This means that every square inch of surface area at sea level supports the equivalent weight of about 14.7 pounds of air above it. To put this in perspective, the total atmospheric pressure on an average human body equals roughly the weight of a small elephant, yet we don't feel crushed because our internal body pressure balances the external atmospheric pressure.
The atmosphere extends hundreds of kilometers above Earth's surface, but its density decreases exponentially with altitude. Approximately 99% of the atmosphere's mass is contained within the first 32 kilometers (20 miles) of altitude, with the majority concentrated in the troposphere—the lowest layer where weather occurs. Understanding atmospheric pressure is crucial for meteorology, aviation, mountaineering, and numerous industrial applications.
Standard Pressure Units
| Unit | Value |
|---|---|
| Hectopascals (hPa) | 1013.25 |
| Millibars (mbar) | 1013.25 |
| Inches Mercury (inHg) | 29.92 |
| Pounds/sq inch (psi) | 14.70 |
| Atmospheres (atm) | 1.000 |