]> Force per unit area. The pressure that a component of a gaseous mixture would have if it alone occupied the same volume at the same temperature as the mixture. The derivative of pressure The pressure exerted by the atmosphere as a consequence of gravitational attraction exerted upon the column of air lying directly above the point in question. The character and amount of atmospheric pressure change during a specified period of time, often a three-hour period preceding an observation. The atmospheric pressure at mean sea level, either directly measured or, most commonly, empirically determined from the observed station pressure. The character and amount of atmospheric pressure change during a specified period of time, often a three-hour period preceding an observation.