What Is Atmospheric Pressure and Why Does It Matter?

Atmospheric pressure is the weight of air above you pressing down on every surface it touches. At sea level, that weight is roughly 101,325 pascals — about 10 tonnes pressing on every square meter of your body. You don't feel it because it acts equally in all directions and your body is adapted to it. But take that pressure away, and the consequences are immediately dramatic.

How We Discovered It

For centuries, nature's "abhorrence of a vacuum" seemed to explain why water rose in suction pumps. Then in 1643, Italian physicist Evangelista Torricelli performed a simple experiment: he filled a glass tube with mercury, inverted it in a bowl of mercury, and watched the column settle at about 760 mm. Nothing held the mercury up on the inside. The weight of air pressing down on the outside mercury surface supported it.

This was the first measurement of atmospheric pressure. The torr (still used in some scientific contexts) is named after Torricelli; 1 torr = 1 mmHg ≈ 133.3 pascals.

Blaise Pascal extended this work, famously showing that atmospheric pressure decreases with altitude by comparing measurements at the base and top of a mountain in France. The SI unit of pressure is named in his honor.

The Numbers

Standard atmospheric pressure at sea level is defined as exactly 101,325 Pa (101.325 kPa, or approximately 14.696 PSI, or 1.01325 bar). In different contexts, different units dominate:

• Meteorology uses hectopascals (hPa) or millibars (1 hPa = 1 mbar). A "normal" weather map shows pressures ranging from about 980 to 1025 hPa.
• Medicine uses millimeters of mercury (mmHg) for blood pressure. Normal blood pressure is around 120/80 mmHg.
• Automotive and industrial contexts in the US use pounds per square inch (PSI). Tire pressure is typically 30-36 PSI.
• Scuba diving uses atmospheres (atm) or bar — pressure increases by 1 atm (roughly 1 bar) for every 10 meters of water depth.

Why Pressure Changes with Altitude

The atmosphere isn't a fixed thickness — it's a column of air that thins with altitude. Half of the atmosphere's mass lies below 5.5 km; 99% lies below 30 km. Every meter you climb, there's slightly less air above you.

At 8,848 meters (the summit of Everest), atmospheric pressure is about 33 kPa — one-third of sea level. Oxygen partial pressure drops proportionally, which is why climbers need supplemental oxygen. Commercial aircraft cabin pressure is maintained at the equivalent of about 1,800-2,400 meters altitude (75-80 kPa), which is why ears pop during ascent and descent.

Pressure and Weather

Atmospheric pressure isn't uniform across the surface. Differences in pressure create wind (air flows from high pressure to low), which drives weather systems. A "low" on a weather map indicates a region of lower-than-average pressure where air is rising, typically bringing clouds and precipitation. A "high" indicates sinking, stable air and fair weather.

The barometer — the instrument Torricelli invented — became the primary tool of meteorology. For centuries, "the glass is falling" (dropping pressure) meant a storm was coming. It still does.

Pressure in Everyday Life

The boiling point of water depends on pressure. At sea level, water boils at 100°C. At altitude, lower pressure means water boils at lower temperatures — around 90°C at 3,000 meters. This is why cooking instructions for pasta or rice include adjustments for high-altitude kitchens; the lower boiling point means food cooks more slowly.

Pressure cookers work in reverse: by sealing the pot and raising internal pressure to about 1.5-2 atm, they raise the boiling point of water to 120°C or more, significantly speeding up cooking.

The column of air above you is heavy, invisible, and consequential. Every time you open a bottle, blow up a tire, or check the weather, you're managing the consequences of that weight.