Does air pressure decrease at higher elevations?

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Dear Tom,

I recently flew from Denver to Chicago. The air pressure at Denver International Airport was 30.04 inches and about the same when I landed at Midway Airport. Shouldn’t the pressure at Denver be much lower, considering it is “the Mile High City?”

— Tommy Milas, Chicago

Dear Tommy,

A column of air of area 1 square inch extending from sea level to the top of the atmosphere weighs about 14.7 pounds. As you ascend into the air (on a jet plane, for instance) less of that column lies above and more lies below you. Therefore the pressure decreases the higher you go — about 0.91 inches lower for every 1,000 feet of ascent. However, readings are “corrected to sea level” so they are comparable at all locations and elevations. A value of 30.04 inches at Denver International Airport, elevation 5,327 feet, is corrected. The actual (uncorrected) pressure there was 25.19 inches.