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What we measure as temperature depends on the collisions of molecules, with temperatures rising as the number of collisions increase. The mass of air molecules in 1 m³ of air at 3000 m is about 0.8 kg, rising to 1.2 kg at sea level (about a 50% increase for a 3000 m loss of elevation; or in the other direction, a 33% decrease for a 3000 m gain in elevation). The increasing density of air means more collisions between molecules, which means a higher temperature. The temperature increase depends on the amount of moisture in the air, and a typical rate would be an increase in temperature of about 1 °C for each 100 m drop in elevation (Figure 2.13). This pattern is referred to as adiabatic heating (or cooling), because the overall energy among the molecules doesn't change even though the temperature changes with air density (or pressure).

FIGURE 2.13 Air temperature increases with decreasing elevation because increasing air density leads to more frequent collisions among air molecules. This adiabatic (no change in energy content) heating depends in part on the moisture content of air, as evaporation or condensation of water moderates the trend that would occur in dry air.

The general relationship graphed in Figure 2.13 can be examined for specific cases, and for details that cause some deviations from the central pattern. Figure 2.14 compares daily high and low temperatures for a valley in the Rocky Mountains of Colorado, USA with corresponding temperatures in the foothills about 50 km away. Rising 1500 m in elevation lowered air pressure by 17%, reducing the summertime average daily highs by about 8 °C, and the average daily lows by 10 °C. The differences in daily lows were smaller in winter, only about 2–3 °C. During the warm season, the lower humidity of the air allowed for a stronger elevation effect than occurred during cool periods when condensation of water moderated temperature changes. Some days in winter were actually warmer at the higher elevation, illustrating the occasional importance of shifting weather systems and cold‐air inversions.

FIGURE 2.14 Daily comparisons of high and low temperatures at locations that differ by 1500 m in elevation showed a general pattern that also included specific days that were higher or lower than the trend. Some of the variation related to the moisture content of the air, especially on colder days, and other variation relates to cloudiness and shifting weather systems.

Source: Data from Steven Fassnacht.