Dear Tom,
I flew from Denver to Chicago and the air pressure at Denver, 30.04 inches, was about the same as it was in Chicago. Shouldn’t Denver’s air pressure be lower than Chicago’s since Denver is much higher?
—Dan Johnston, Oak Brook
Dear Dan,
The standard pressure of air at the bottom of a column extending from sea level to the “top” of the atmosphere is 14.7 pounds per square inch. As you noted, the mass of the atmosphere decreases as you ascend, thus air pressure also decreases with increasing elevation. That decrease is about 0.91 inch for every 1,000 ft. of altitude. Air pressure is usually given in units of sea level pressure (SLP). Readings are “reduced to sea level” so that they are comparable at all locations. A value of 30.04 inches at Denver International Airport, elevation 5,327 feet, is its SLP. The actual pressure at station elevation would be 25.19”.
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