Atmospheric pressure, also known as air pressure or barometric pressure, is the pressure within the atmosphere of Earth. It is the force exerted on a surface by the air above it as gravity pulls it to Earth. The standard unit of atmospheric pressure is the Pascal (Pa), but it is commonly measured in other units such as millibars (mb) or pounds per square inch (psi) . The standard atmosphere (symbol: atm) is a unit of pressure defined as 101,325 Pa, which is equivalent to 1,013.25 millibars, 760 mm Hg.

Atmospheric pressure is caused by the gravitational attraction of the planet on the atmospheric gases above the surface and is a function of the mass of the planet, the radius of the surface, and the amount and composition of the gases and their vertical distribution in the atmosphere. It is modified by the planetary rotation and local effects such as wind velocity, density variations due to temperature, and variations in composition. Mean sea-level pressure is the atmospheric pressure at sea level, and it is used as a reference point for measuring atmospheric pressure.

Changes in atmospheric pressure can indicate changes in weather patterns and forecasts around the world. A rising barometer indicates increasing atmospheric pressure, while a falling barometer indicates decreasing atmospheric pressure. When you go up into the mountains or fly high in an airplane, the air is thinner, and the pressure is lower. Conversely, the pressure increases as you go deeper into the ocean.

The typical pressure at sea level is 1013.25 millibars or 14.7 pounds per square inch. One atmosphere (101.325 kPa or 14.7 psi) is also the pressure caused by the weight of a column of freshwater of approximately 10.3 m (33.8 ft) . Thus, a diver 10.3 m underwater experiences a pressure of about 2 atmospheres (1 atm of air plus 1 atm of water) .

In summary, atmospheric pressure is the force exerted by the weight of the air above a surface due to gravity. It is measured in units such as millibars or pounds per square inch and is affected by factors such as altitude, temperature, and weather patterns.