The layer of gases that makes up Earth's atmosphere is a number of miles thick. We would feel this weight like a stack of bricks on our head except that a body of air, like a body of water, produces fluid pressure that acts in all directions. Probably the only time we are aware of this pressure is when it is not balanced within our bodies - across our eardrums when landing in an airplane, for instance. Fluid pressure caused by the weight of atmospheric gases is called atmospheric, or barometric, pressure. Barometric pressure is primarily determined by the thickness and density of the atmosphere above a given point on Earth.
Barometric pressure has several implications in water quality monitoring.
First, barometric pressure can determine the extent to which atmospheric gases can be dissolved in water. Less atmospheric gases can be dissolved in water in Denver than in San Diego or Miami because Denver, being a mile higher than coastal cities, has a thinner atmosphere and so a lower ambient barometric pressure. Second, barometric pressure measurement is important in calibrating dissolved oxygen sensors. Lower barometric pressures imply lower equilibrium oxygen concentrations. Post-calibration changes in barometric pressure may be used to correct percent saturation readings.