Atmospheric Pressure Explained: What It Is and Why It Changes

The standard atmosphere (atm) is a defined reference point: exactly 101,325 Pa = 101.325 kPa = 1.01325 bar = 14.696 PSI = 760 mmHg = 760 torr. This represents the average sea-level air pressure and is used as a baseline in chemistry, physics, diving, aviation, and engineering calculations. It is a defined constant, not a measured average — real atmospheric pressure varies around this reference.

In meteorology, pressure is reported in hectopascals (hPa) or millibars (mbar), both equal to 100 Pa. Standard sea-level pressure = 1013.25 hPa. High-pressure systems (associated with clear weather) typically show 1020–1030 hPa; low-pressure systems (storms) fall below 1000 hPa. Severe hurricanes can drop to 900 hPa at the center.

Atmospheric pressure decreases with altitude because there is less air mass above. The relationship is roughly exponential: pressure drops by about 12 Pa for every 1 meter of elevation near sea level. At 5,500 m (18,000 ft) — roughly half the atmosphere's mass is below — pressure is about 500 hPa (50 kPa), half of sea level. At the cruising altitude of a commercial jet (35,000 ft / 10,700 m), outside pressure is about 23 kPa (3.4 PSI), which is why cabin pressurization is essential.

Practical altitude effects: water boils at 100°C at sea level (101.3 kPa) but only at 95°C in Denver (about 84 kPa, 5,280 ft elevation) and 89°C at 3,000 m. Cooking times increase at altitude. Bread rises faster. Barometric altimeters in aircraft measure altitude indirectly from pressure, which is why a local pressure setting (QNH) must be dialed in for accurate altitude readings.

Barometers measure atmospheric pressure. Traditional mercury barometers measure the height of a mercury column supported by atmospheric pressure — at standard pressure, mercury rises 760 mm (29.92 inches), which is where mmHg and inHg come from as pressure units. Aneroid barometers use a sealed flexible capsule that expands and contracts with pressure changes; these are portable and used in weather stations and aircraft altimeters.

Digital barometers use piezoelectric or MEMS pressure sensors. Most modern smartphones include a barometric pressure sensor, used for altitude estimation and indoor navigation. Weather stations report mean sea-level pressure (MSLP), which adjusts the local barometric reading to what it would be at sea level — this allows fair comparison between stations at different altitudes.