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Mountains and Rain Shadows

The Air Rises and Cools

As air carrying moisture moves toward a mountain range, the slope of the mountain range forces the air upward (see diagram on the right).

The diagram on the left illustrates how the air pressure higher up in the atmosphere is lower because fewer molecules push down from above. Therefore, as the moisture-filled air rises, it expands as the air pressure decreases. As the molecules in the air move further apart, they lose energy and slow down, which lowers the temperature.
An illustration shows how the density of air molecules decreases as altitude increases.

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A vertical column contains densely packed molecules at the bottom, labeled high pressure. The molecules become more spread out higher up in the column. At the top, labeled low pressure, there are just a few occasional molecules.
An illustration showing the same cross section of a mountain range. An arrow indicates air flowing up the left side of the mountains from sea level toward the peaks.

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Temperature typically decreases about 5.5°F for every 1,000 feet of altitude gained. For example, if this mountain peak is 8,000 feet, the temperature at the peak would be 44°F cooler than at the bottom of the mountain!

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