What is the “lapse rate”?

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Dear Tom,
What is the “lapse rate”? I have seen that term occasionally.
— Fred Manse
Dear Fred,
When the pressure of dry air changes, its temperature also changes (by “dry air,” the meaning is no clouds or fog). Changes in altitude equate to changes in air pressure.
When a parcel of air rises, its pressure drops and its temperature is reduced by 5.4 degrees for every 1,000 feet of ascent. This assumes no condensation has taken place. Similarly, when air moves downward, its pressure increases and its temperature rises by 5.4 degrees for every 1,000 feet of descent. This change of temperature is called the dry adiabatic lapse rate.
Here’s an example: At the top of a mountain peak at an elevation of 10,000 feet, the temperature would be 54 degrees lower than the temperature at sea level (this assumes no clouds or fog).