Are drops in air pressure, temperature related where rising altitude is concerned?

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Dear Tom,

Recently you stated that at an altitude of 18,500 feet, the air pressure decreases to about half of the sea level pressure and also that the temperature drops 5.4 degrees for every 1,000 feet of elevation increase. Are these two facts related?

— Lou Leibovich, Hoffman Estates

Dear Lou,

They are unrelated, but the implication is that air at a temperature of 100 degrees at sea level, if it is dry, will have its temperature reduced to zero degrees at 18,500 feet because the temperature of dry air falls 5.4 degrees for every 1,000 feet of elevation increase.

For example, this explains why glaciers and snowfields exist at the summit of Mt. Kilimanjaro, elevation about 19,341 feet, even though Kilimanjaro, in Africa, is located approximately 200 miles from the Equator.

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