What is barometric pressure?

Barometric pressure refers to the weight of air above a certain surface, typically measured in terms of atmospheric pressure at a given altitude. This weight is a result of the combined mass of air molecules in the atmosphere pressing down due to gravity. At sea level, standard barometric pressure is approximately 1013.25 hPa (hectopascals) or 29.92 inches of mercury.

This concept is crucial in understanding how atmospheric pressure changes with altitude, and influences various aspects of aerospace physiology, including how oxygen availability decreases as one ascends into thinner air. The other choices pertain to different concepts; they do not accurately define barometric pressure, which specifically relates to the weight of air above a given reference point.