airpressure

Airpressure, usually referred to as air pressure or atmospheric pressure, is the force per unit area exerted by the weight of the Earth's atmosphere on surfaces. It arises from the constant collisions of air molecules with surfaces and with each other. Pressure is commonly measured in pascals (Pa), with larger readings expressed as kilopascals (kPa); older units include millimeters of mercury (mmHg) and inches of mercury (inHg). At sea level, standard atmospheric pressure is about 101,325 Pa (101.3 kPa, 760 mmHg, 29.92 inHg).

Pressure can be described as absolute, gauge, or differential. Absolute pressure is measured relative to a vacuum; gauge pressure is the difference between absolute pressure and the surrounding atmospheric pressure; differential pressure is the difference between two pressures. In many practical contexts, gauge pressure is used.

Air pressure varies with altitude: it decreases as elevation increases because the air becomes less dense higher up. In the standard atmosphere, pressure declines from about 1013 hPa at sea level to much lower values with increasing height. Weather systems also cause pressure variations: high-pressure areas are typically associated with clearer skies, while low-pressure areas bring clouds and precipitation. These variations influence wind, cloud formation, and the behavior of engines and aircraft.

Measuring air pressure relies on barometers. Mercury barometers use a column of mercury balanced by atmospheric pressure, while aneroid barometers use sealed, flexible membranes. Altimeters estimate altitude by translating ambient pressure changes into height. The relationship between pressure, temperature, and density is described by the ideal gas law, P = ρRT.

Airpressure is central to meteorology, aviation, physiology, and many industrial processes, and is reported in standard units to enable consistent communication across disciplines.