What is atmospheric pressure defined as?

Atmospheric pressure is defined as the weight of the air surrounding the Earth. This pressure is generated by the gravitational pull of the Earth on the air molecules, which creates a column of air exerting force on any given area at the Earth's surface. At sea level, this weight of the air produces an average atmospheric pressure of approximately 1013.25 hPa (hectopascals) or millibars.

Understanding that atmospheric pressure results from the weight of air helps clarify how it varies with altitude. As you ascend, there is less air above you, which reduces the weight of the air column acting on you, thus decreasing the atmospheric pressure. This concept is critical for divers, as changes in pressure affect buoyancy and gas absorption.

Considering the other choices, the density of air at sea level relates to how closely packed air molecules are but does not directly define pressure. The force exerted by air on a specific area, while a valid aspect of pressure measurement, does not encapsulate the concept of atmospheric pressure in terms of its origin. Lastly, the amount of air needed for breathing pertains more to biological requirements rather than a physical definition of atmospheric pressure. Hence, the focus on the weight of the air provides a clear understanding of what atmospheric