What does it mean when the barometer reads exactly 30 inches?

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Dear Carl,
What does it mean when the barometer reads exactly 30 inches?
— Carl Dixon
Dear Carl,
Air pressure is the weight of the Earth’s atmosphere upon every object. It is the total weight of all the air above that object. Because the amount of air above an object decreases as that object rises, air pressure values taken at various elevations are “reduced to sea level” — changed to the value that would occur if that object were at sea level.

At sea level, air pressure averages 29.92 inches of mercury. That means a glass tube, with its air pumped out and sealed at its upper end and placed in a dish of mercury at its lower (open) end, would have mercury pushed by air pressure up to a height of 29.92 inches in the tube. An air pressure of 30 inches means the mercury will have risen to a height of exactly 30 inches.

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