Mountains and the Water Cycle: the rain shadow effect explained

Here in the Rocky Mountains, we are approaching an important time of the year: spring snowmelt! Throughout the winter, snow builds up in the mountains, then when warmer weather comes, it rapidly melts, causing a massive increase in the flows of streams and rivers. The water that starts in the mountains benefits all sorts of downstream users – fish like salmon that travel dozens or even hundreds of miles through rivers, riparian plants like willows and aspen that thrive near streams, and all sorts of animals that need water for drinking or habitat purposes. And of course, people use this water – from mountain towns to coastal cities and anywhere in between! Scientists have aptly described mountains as the water towers of the world. Whether you live near mountains or not, water resources near you are likely impacted by the seasonal cycle of snow building up and melting in mountains. 

But why do mountains get so much precipitation, to begin with? The answer relates to the water cycle and temperature. When air comes off of the ocean and starts moving across a continent, it is warm and has lots of water vapor. As that air moves inland and encounters mountains, it must move upwards to pass over the mountains. You may recall from my recent post on the water cycle that as water vapor rises up in the atmosphere it cools and condenses into liquid water droplets. As the tall mountains force air to travel higher, the cooler temperatures at greater elevations cause water vapor to condense and fall as raindrops or snowflakes, depending on the temperature.

The side of the mountain where the air travels up first will receive lots of precipitation. As air moves lower down the opposite side of the mountain, it becomes warmer again. But now, that air has less vapor than before because it lost water as precipitation while traveling over the mountains. The result is warmer, drier air on the inland side of the mountains. This is why we often see lots of precipitation on one side of mountains and arid regions or deserts on the other side. This noticeable difference is called the rain shadow effect. 

In the Rocky Mountains, air generally moves eastward: traveling from the Pacific Ocean and moving east across North America. This is why the western slope of the Rocky Mountains experiences more snow than the Front Range on the east side. However, demand for water is high on the east side of the mountains, where many people live in cities such as Denver and Colorado Springs. To keep up with the demand for water, humans actually move water from the west side to the east side of the Rocky Mountains using man-made ditches high in the mountains, that catch and divert snowmelt from where it would naturally flow. Where humans live doesn’t always match up with where water resources are available. This motivates us to alter the water cycle to meet our needs…

Continue reading about the ways humans impact the water cycle.

Or learn about how the water cycle works.

Or download and print a water cycle poster!

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References and Further Reading

1. Kennedy, C. (2011). Rain shadows on the summits of Hawaii. NOAA Climate.gov. Available: https://www.climate.gov/news-features/featured-images/rain-shadows-summits-hawaii
2. Plummer, C. C., Carlson, D. H., & Hammersley L. (2019) Physical Geology. New York, NY: McGraw-Hill Education. (16th ed., pp. 232 – 320).

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